The biggest trap of all is forgetting to use warnings or use the -wswitch; see perllexwarn and perlrun. The second biggest trap is notmaking your entire program runnable under use strict. The third biggesttrap is not reading the list of changes in this version of Perl; seeperldelta.

Awk Traps

Accustomed awk users should take special note of the following:

A Perl program executes only once, not once for each input line. You cando an implicit loop with -n or -p.
The English module, loaded via
```
 use English;
 
```
allows you to refer to special variables (like $/) with names (like$RS), as though they were in awk; see perlvar for details.
Semicolons are required after all simple statements in Perl (exceptat the end of a block). Newline is not a statement delimiter.
Curly brackets are required on ifs and whiles.
Variables begin with "$", "@" or "%" in Perl.
Arrays index from 0. Likewise string positions in substr() andindex().
You have to decide whether your array has numeric or string indices.
Hash values do not spring into existence upon mere reference.
You have to decide whether you want to use string or numericcomparisons.
Reading an input line does not split it for you. You get to split itto an array yourself. And the split() operator has differentarguments than awk's.
The current input line is normally in $_, not $0. It generally doesnot have the newline stripped. ($0 is the name of the programexecuted.) See perlvar.
$<digit> does not refer to fields--it refers to substrings matchedby the last match pattern.
The print() statement does not add field and record separators unlessyou set $, and $\. You can set $OFS and $ORS if you're usingthe English module.
You must open your files before you print to them.
The range operator is "..", not comma. The comma operator works as inC.
The match operator is "=~", not "~". ("~" is the one's complementoperator, as in C.)
The exponentiation operator is "**", not "^". "^" is the XORoperator, as in C. (You know, one could get the feeling that awk isbasically incompatible with C.)
The concatenation operator is ".", not the null string. (Using thenull string would render /pat/ /pat/ unparsable, because the third slashwould be interpreted as a division operator--the tokenizer is in factslightly context sensitive for operators like "/", "?", and ">".And in fact, "." itself can be the beginning of a number.)
The next, exit, and continue keywords work differently.

The following variables work differently:

  AwkPerl
   ARGCscalar @ARGV (compare with $#ARGV)
   ARGV[0]$0
   FILENAME$ARGV
   FNR$. - something
   FS(whatever you like)
   NF$#Fld, or some such
   NR$.
   OFMT$#
   OFS$,
   ORS$\
   RLENGTHlength($&)
   RS$/
   RSTARTlength($`)
   SUBSEP$;

You cannot set $RS to a pattern, only a string.
When in doubt, run the awk construct through a2p and see what itgives you.

C/C++ Traps

Cerebral C and C++ programmers should take note of the following:

Curly brackets are required on if's and while's.
You must use elsif rather than else if.
The break and continue keywords from C become in Perl lastand next, respectively. Unlike in C, these do not work within ado { } while construct. See Loop Control in perlsyn.
The switch statement is called given/when and only available inperl 5.10 or newer. See Switch Statements in perlsyn.
Variables begin with "$", "@" or "%" in Perl.
Comments begin with "#", not "/*" or "//". Perl may interpret C/C++comments as division operators, unterminated regular expressions orthe defined-or operator.
You can't take the address of anything, although a similar operatorin Perl is the backslash, which creates a reference.
ARGV must be capitalized. $ARGV[0] is C's argv[1], and argv[0]ends up in $0.
System calls such as link(), unlink(), rename(), etc. return nonzero forsuccess, not 0. (system(), however, returns zero for success.)
Signal handlers deal with signal names, not numbers. Use kill -lto find their names on your system.

Sed Traps

Seasoned sed programmers should take note of the following:

A Perl program executes only once, not once for each input line. You cando an implicit loop with -n or -p.
Backreferences in substitutions use "$" rather than "\".
The pattern matching metacharacters "(", ")", and "|" do not have backslashesin front.
The range operator is ..., rather than comma.

Shell Traps

Sharp shell programmers should take note of the following:

The backtick operator does variable interpolation without regard tothe presence of single quotes in the command.
The backtick operator does no translation of the return value, unlike csh.
Shells (especially csh) do several levels of substitution on eachcommand line. Perl does substitution in only certain constructssuch as double quotes, backticks, angle brackets, and search patterns.
Shells interpret scripts a little bit at a time. Perl compiles theentire program before executing it (except for BEGIN blocks, whichexecute at compile time).
The arguments are available via @ARGV, not $1, $2, etc.
The environment is not automatically made available as separate scalarvariables.
The shell's test uses "=", "!=", "<" etc for string comparisons and "-eq","-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, whichuses eq, ne, lt for string comparisons, and ==, != < etcfor numeric comparisons.

Perl Traps

Practicing Perl Programmers should take note of the following:

Remember that many operations behave differently in a listcontext than they do in a scalar one. See perldata for details.
Avoid barewords if you can, especially all lowercase ones.You can't tell by just looking at it whether a bareword isa function or a string. By using quotes on strings andparentheses on function calls, you won't ever get them confused.
You cannot discern from mere inspection which builtinsare unary operators (like chop() and chdir())and which are list operators (like print() and unlink()).(Unless prototyped, user-defined subroutines can only be listoperators, never unary ones.) See perlop and perlsub.
People have a hard time remembering that some functionsdefault to $_, or @ARGV, or whatever, but that others whichyou might expect to do not.
The <FH> construct is not the name of the filehandle, it is a readlineoperation on that handle. The data read is assigned to $_ only if thefile read is the sole condition in a while loop:
```
 while (<FH>)  { }
  while (defined($_ = <FH>)) { }..
  <FH>;  # data discarded!
 
```
Remember not to use = when you need =~;these two constructs are quite different:
```
 $x =  /foo/;
  $x =~ /foo/;
 
```
The do {} construct isn't a real loop that you can useloop control on.
Use my() for local variables whenever you can get away withit (but see perlform for where you can't).Using local() actually gives a local value to a globalvariable, which leaves you open to unforeseen side-effectsof dynamic scoping.
If you localize an exported variable in a module, its exported value willnot change. The local name becomes an alias to a new value but theexternal name is still an alias for the original.

Perl4 to Perl5 Traps

Practicing Perl4 Programmers should take note of the followingPerl4-to-Perl5 specific traps.

They're crudely ordered according to the following list:

Discontinuance, Deprecation, and BugFix traps
Anything that's been fixed as a perl4 bug, removed as a perl4 featureor deprecated as a perl4 feature with the intent to encourage usage ofsome other perl5 feature.
Parsing Traps
Traps that appear to stem from the new parser.
Numerical Traps
Traps having to do with numerical or mathematical operators.
General data type traps
Traps involving perl standard data types.
Context Traps - scalar, list contexts
Traps related to context within lists, scalar statements/declarations.
Precedence Traps
Traps related to the precedence of parsing, evaluation, and execution ofcode.
General Regular Expression Traps using s///, etc.
Traps related to the use of pattern matching.
Subroutine, Signal, Sorting Traps
Traps related to the use of signals and signal handlers, general subroutines,and sorting, along with sorting subroutines.
OS Traps
OS-specific traps.
DBM Traps
Traps specific to the use of dbmopen(), and specific dbm implementations.
Unclassified Traps
Everything else.

If you find an example of a conversion trap that is not listed here,please submit it to <[email protected]> for inclusion.Also note that at least some of these can be caught with theuse warnings pragma or the -w switch.

Discontinuance, Deprecation, and BugFix traps

Anything that has been discontinued, deprecated, or fixed asa bug from perl4.

Symbols starting with "_" no longer forced into main

Symbols starting with "_" are no longer forced into package main, exceptfor $_ itself (and @_, etc.).

 package test;
  $_legacy = 1;
 
  package main;
  print "\$_legacy is ",$_legacy,"\n";
 
  # perl4 prints: $_legacy is 1
  # perl5 prints: $_legacy is

Double-colon valid package separator in variable name
Double-colon is now a valid package separator in a variable name. Thus thesebehave differently in perl4 vs. perl5, because the packages don't exist.
```
 $a=1;$b=2;$c=3;$var=4;
  print "$a::$b::$c ";
  print "$var::abc::xyz\n";
 
  # perl4 prints: 1::2::3 4::abc::xyz
  # perl5 prints: 3
 
```
Given that :: is now the preferred package delimiter, it is debatablewhether this should be classed as a bug or not.(The older package delimiter, ' ,is used here)
```
 $x = 10;
  print "x=${'x}\n";
 
  # perl4 prints: x=10
  # perl5 prints: Can't find string terminator "'" anywhere before EOF
 
```
You can avoid this problem, and remain compatible with perl4, if youalways explicitly include the package name:
```
 $x = 10;
  print "x=${main'x}\n";
 
```
Also see precedence traps, for parsing $:.

2nd and 3rd args to splice() are now in scalar context

The second and third arguments of splice() are now evaluated in scalarcontext (as the Camel says) rather than list context.

 sub sub1{return(0,2) }  # return a 2-element list
  sub sub2{ return(1,2,3)} # return a 3-element list
  @a1 = ("a","b","c","d","e");
  @a2 = splice(@a1,&sub1,&sub2);
  print join(' ',@a2),"\n";
 
  # perl4 prints: a b
  # perl5 prints: c d e

Can't do goto into a block that is optimized away

You can't do a goto into a block that is optimized away. Darn.

 goto marker1;
 
  for(1){
  marker1:
  print "Here I is!\n";
  }
 
  # perl4 prints: Here I is!
  # perl5 errors: Can't "goto" into the middle of a foreach loop

Can't use whitespace as variable name or quote delimiter
It is no longer syntactically legal to use whitespace as the nameof a variable, or as a delimiter for any kind of quote construct.Double darn.
```
 $a = ("foo bar");
  $b = q baz ;
  print "a is $a, b is $b\n";
 
  # perl4 prints: a is foo bar, b is baz
  # perl5 errors: Bareword found where operator expected
 
```

while/if BLOCK BLOCK gone

The archaic while/if BLOCK BLOCK syntax is no longer supported.

 if { 1 } {
  print "True!";
  }
  else {
  print "False!";
  }
 
  # perl4 prints: True!
  # perl5 errors: syntax error at test.pl line 1, near "if {"

** binds tighter than unary minus
The ** operator now binds more tightly than unary minus.It was documented to work this way before, but didn't.
```
 print -4**2,"\n";
 
  # perl4 prints: 16
  # perl5 prints: -16
 
```
foreach changed when iterating over a list
The meaning of foreach{} has changed slightly when it is iterating over alist which is not an array. This used to assign the list to atemporary array, but no longer does so (for efficiency). This meansthat you'll now be iterating over the actual values, not over copies ofthe values. Modifications to the loop variable can change the originalvalues.
```
 @list = ('ab','abc','bcd','def');
  foreach $var (grep(/ab/,@list)){
  $var = 1;
  }
  print (join(':',@list));
 
  # perl4 prints: ab:abc:bcd:def
  # perl5 prints: 1:1:bcd:def
 
```
To retain Perl4 semantics you need to assign your listexplicitly to a temporary array and then iterate over that. Forexample, you might need to change
```
 foreach $var (grep(/ab/,@list)){
 
```
to
```
 foreach $var (@tmp = grep(/ab/,@list)){
 
```
Otherwise changing $var will clobber the values of @list. (This most oftenhappens when you use $_ for the loop variable, and call subroutines inthe loop that don't properly localize $_.)
split with no args behavior changed
split with no arguments now behaves like split ' ' (which doesn'treturn an initial null field if $_ starts with whitespace), it used tobehave like split /\s+/ (which does).
```
 $_ = ' hi mom';
  print join(':', split);
 
  # perl4 prints: :hi:mom
  # perl5 prints: hi:mom
 
```
-e behavior fixed
Perl 4 would ignore any text which was attached to an -e switch,always taking the code snippet from the following arg. Additionally, itwould silently accept an -e switch without a following arg. Both ofthese behaviors have been fixed.
```
 perl -e'print "attached to -e"' 'print "separate arg"'
 
  # perl4 prints: separate arg
  # perl5 prints: attached to -e
 
  perl -e
 
  # perl4 prints:
  # perl5 dies: No code specified for -e.
 
```
push returns number of elements in resulting list
In Perl 4 the return value of push was undocumented, but it wasactually the last value being pushed onto the target list. In Perl 5the return value of push is documented, but has changed, it is thenumber of elements in the resulting list.
```
 @x = ('existing');
  print push(@x, 'first new', 'second new');
 
  # perl4 prints: second new
  # perl5 prints: 3
 
```
Some error messages differ
Some error messages will be different.
split() honors subroutine args
In Perl 4, if in list context the delimiters to the first argument ofsplit() were ??, the result would be placed in @_ as well asbeing returned. Perl 5 has more respect for your subroutine arguments.
Bugs removed
Some bugs may have been inadvertently removed. :-)

Parsing Traps

Perl4-to-Perl5 traps from having to do with parsing.

Space between . and = triggers syntax error

Note the space between . and =

 $string . = "more string";
  print $string;
 
  # perl4 prints: more string
  # perl5 prints: syntax error at - line 1, near ". ="

Better parsing in perl 5

Better parsing in perl 5

 sub foo {}
  &foo
  print("hello, world\n");
 
  # perl4 prints: hello, world
  # perl5 prints: syntax error

Function parsing

"if it looks like a function, it is a function" rule.

  print
  ($foo == 1) ? "is one\n" : "is zero\n";
 
  # perl4 prints: is zero
  # perl5 warns: "Useless use of a constant in void context" if using -w

String interpolation of $#array differs

String interpolation of the $#array construct differs when bracesare to used around the name.

 @a = (1..3);
  print "${#a}";
 
  # perl4 prints: 2
  # perl5 fails with syntax error
 
  @a = (1..3);
  print "$#{a}";
 
  # perl4 prints: {a}
  # perl5 prints: 2

Perl guesses on map, grep followed by { if it starts BLOCK or hash ref
When perl sees map { (or grep {), it has to guess whether the {starts a BLOCK or a hash reference. If it guesses wrong, it will reporta syntax error near the } and the missing (or unexpected) comma.
Use unary + before { on a hash reference, and unary + appliedto the first thing in a BLOCK (after {), for perl to guess right allthe time. (See map.)

Numerical Traps

Perl4-to-Perl5 traps having to do with numerical operators,operands, or output from same.

Formatted output and significant digits
Formatted output and significant digits. In general, Perl 5tries to be more precise. For example, on a Solaris Sparc:
```
 print 7.373504 - 0, "\n";
  printf "%20.18f\n", 7.373504 - 0;
 
  # Perl4 prints:
  7.3750399999999996141
  7.375039999999999614
 
  # Perl5 prints:
  7.373504
  7.373503999999999614
 
```
Notice how the first result looks better in Perl 5.
Your results may vary, since your floating point formatting routinesand even floating point format may be slightly different.
Auto-increment operator over signed int limit deleted
This specific item has been deleted. It demonstrated how the auto-incrementoperator would not catch when a number went over the signed int limit. Fixedin version 5.003_04. But always be wary when using large integers.If in doubt:
```
   use Math::BigInt;
 
```
Assignment of return values from numeric equality tests doesn't work
Assignment of return values from numeric equality testsdoes not work in perl5 when the test evaluates to false (0).Logical tests now return a null, instead of 0
```
 $p = ($test == 1);
  print $p,"\n";
 
  # perl4 prints: 0
  # perl5 prints:
 
```
Also see General Regular Expression Traps using s///, etc.for another example of this new feature...
Bitwise string ops
When bitwise operators which can operate upon either numbers orstrings (& | ^ ~) are given only strings as arguments, perl4 wouldtreat the operands as bitstrings so long as the program contained a callto the vec() function. perl5 treats the string operands as bitstrings.(See Bitwise String Operators in perlop for more details.)
```
 $fred = "10";
  $barney = "12";
  $betty = $fred & $barney;
  print "$betty\n";
  # Uncomment the next line to change perl4's behavior
  # ($dummy) = vec("dummy", 0, 0);
 
  # Perl4 prints:
  8
 
  # Perl5 prints:
  10
 
  # If vec() is used anywhere in the program, both print:
  10
 
```

General data type traps

Perl4-to-Perl5 traps involving most data-types, and their usagewithin certain expressions and/or context.

Negative array subscripts now count from the end of array

Negative array subscripts now count from the end of the array.

 @a = (1, 2, 3, 4, 5);
  print "The third element of the array is $a[3] also expressed as $a[-2] \n";
 
  # perl4 prints: The third element of the array is 4 also expressed as
  # perl5 prints: The third element of the array is 4 also expressed as 4

Setting $#array lower now discards array elements

Setting $#array lower now discards array elements, and makes themimpossible to recover.

 @a = (a,b,c,d,e);
  print "Before: ",join('',@a);
  $#a =1;
  print ", After: ",join('',@a);
  $#a =3;
  print ", Recovered: ",join('',@a),"\n";
 
  # perl4 prints: Before: abcde, After: ab, Recovered: abcd
  # perl5 prints: Before: abcde, After: ab, Recovered: ab

Hashes get defined before use

Hashes get defined before use

 local($s,@a,%h);
  die "scalar \$s defined" if defined($s);
  die "array \@a defined" if defined(@a);
  die "hash \%h defined" if defined(%h);
 
  # perl4 prints:
  # perl5 dies: hash %h defined

Perl will now generate a warning when it sees defined(@a) anddefined(%h).

Glob assignment from localized variable to variable
glob assignment from variable to variable will fail if the assignedvariable is localized subsequent to the assignment
```
 @a = ("This is Perl 4");
  *b = *a;
  local(@a);
  print @b,"\n";
 
  # perl4 prints: This is Perl 4
  # perl5 prints:
 
```
Assigning undef to glob
Assigning undef to a glob has no effect in Perl 5. In Perl 4it undefines the associated scalar (but may have other side effectsincluding SEGVs). Perl 5 will also warn if undef is assigned to atypeglob. (Note that assigning undef to a typeglob is differentthan calling the undef function on a typeglob (undef *foo), whichhas quite a few effects.
```
 $foo = "bar";
  *foo = undef;
  print $foo;
 
  # perl4 prints:
  # perl4 warns: "Use of uninitialized variable" if using -w
  # perl5 prints: bar
  # perl5 warns: "Undefined value assigned to typeglob" if using -w
 
```
Changes in unary negation (of strings)
Changes in unary negation (of strings)This change effects both the return value and what itdoes to auto(magic)increment.
```
 $x = "aaa";
  print ++$x," : ";
  print -$x," : ";
  print ++$x,"\n";
 
  # perl4 prints: aab : -0 : 1
  # perl5 prints: aab : -aab : aac
 
```

Modifying of constants prohibited

perl 4 lets you modify constants:

 $foo = "x";
  &mod($foo);
  for ($x = 0; $x < 3; $x++) {
  &mod("a");
  }
  sub mod {
  print "before: $_[0]";
  $_[0] = "m";
  print "  after: $_[0]\n";
  }
 
  # perl4:
  # before: x  after: m
  # before: a  after: m
  # before: m  after: m
  # before: m  after: m
 
  # Perl5:
  # before: x  after: m
  # Modification of a read-only value attempted at foo.pl line 12.
  # before: a

defined $var behavior changed

The behavior is slightly different for:

 print "$x", defined $x
 
  # perl 4: 1
  # perl 5: <no output, $x is not called into existence>

Variable Suicide

Variable suicide behavior is more consistent under Perl 5.Perl5 exhibits the same behavior for hashes and scalars,that perl4 exhibits for only scalars.

 $aGlobal{ "aKey" } = "global value";
  print "MAIN:", $aGlobal{"aKey"}, "\n";
  $GlobalLevel = 0;
  &test( *aGlobal );
 
  sub test {
  local( *theArgument ) = @_;
  local( %aNewLocal ); # perl 4 != 5.001l,m
  $aNewLocal{"aKey"} = "this should never appear";
  print "SUB: ", $theArgument{"aKey"}, "\n";
  $aNewLocal{"aKey"} = "level $GlobalLevel";   # what should print
  $GlobalLevel++;
  if( $GlobalLevel<4 ) {
  &test( *aNewLocal );
  }
  }
 
  # Perl4:
  # MAIN:global value
  # SUB: global value
  # SUB: level 0
  # SUB: level 1
  # SUB: level 2
 
  # Perl5:
  # MAIN:global value
  # SUB: global value
  # SUB: this should never appear
  # SUB: this should never appear
  # SUB: this should never appear

Context Traps - scalar, list contexts

Elements of argument lists for formats evaluated in list context

The elements of argument lists for formats are now evaluated in listcontext. This means you can interpolate list values now.

 @fmt = ("foo","bar","baz");
  format STDOUT=
  @<<<<< @||||| @>>>>>
  @fmt;
  .
  write;
 
  # perl4 errors:  Please use commas to separate fields in file
  # perl5 prints: foo bar  baz

caller() returns false value in scalar context if no caller present
The caller() function now returns a false value in a scalar contextif there is no caller. This lets library files determine if they'rebeing required.
```
 caller() ? (print "You rang?\n") : (print "Got a 0\n");
 
  # perl4 errors: There is no caller
  # perl5 prints: Got a 0
 
```
Comma operator in scalar context gives scalar context to args
The comma operator in a scalar context is now guaranteed to give ascalar context to its last argument. It gives scalar or void contextto any preceding arguments, depending on circumstances.
```
 @y= ('a','b','c');
  $x = (1, 2, @y);
  print "x = $x\n";
 
  # Perl4 prints:  x = c   # Interpolates array @y into the list
  # Perl5 prints:  x = 3   # Evaluates array @y in scalar context
 
```

sprintf() prototyped as ($;@)

sprintf() is prototyped as ($;@), so its first argument is given scalarcontext. Thus, if passed an array, it will probably not do what you want,unlike Perl 4:

 @z = ('%s%s', 'foo', 'bar');
  $x = sprintf(@z);
  print $x;
 
  # perl4 prints: foobar
  # perl5 prints: 3

printf() works the same as it did in Perl 4, though:

 @z = ('%s%s', 'foo', 'bar');
  printf STDOUT (@z);
 
  # perl4 prints: foobar
  # perl5 prints: foobar

Precedence Traps

Perl4-to-Perl5 traps involving precedence order.

Perl 4 has almost the same precedence rules as Perl 5 for the operatorsthat they both have. Perl 4 however, seems to have had someinconsistencies that made the behavior differ from what was documented.

LHS vs. RHS of any assignment operator
LHS vs. RHS of any assignment operator. LHS is evaluated firstin perl4, second in perl5; this can affect the relationshipbetween side-effects in sub-expressions.
```
 @arr = ( 'left', 'right' );
  $a{shift @arr} = shift @arr;
  print join( ' ', keys %a );
 
  # perl4 prints: left
  # perl5 prints: right
 
```

Semantic errors introduced due to precedence

These are now semantic errors because of precedence:

 @list = (1,2,3,4,5);
  %map = ("a",1,"b",2,"c",3,"d",4);
  $n = shift @list + 2;   # first item in list plus 2
  print "n is $n, ";
  $m = keys %map + 2; # number of items in hash plus 2
  print "m is $m\n";
 
  # perl4 prints: n is 3, m is 6
  # perl5 errors and fails to compile

Precedence of assignment operators same as the precedence of assignment
The precedence of assignment operators is now the same as the precedenceof assignment. Perl 4 mistakenly gave them the precedence of the associatedoperator. So you now must parenthesize them in expressions like
```
 /foo/ ? ($a += 2) : ($a -= 2);
 
```
Otherwise
```
 /foo/ ? $a += 2 : $a -= 2
 
```
would be erroneously parsed as
```
 (/foo/ ? $a += 2 : $a) -= 2;
 
```
On the other hand,
```
 $a += /foo/ ? 1 : 2;
 
```
now works as a C programmer would expect.
open requires parentheses around filehandle
```
 open FOO || die;
 
```
is now incorrect. You need parentheses around the filehandle.Otherwise, perl5 leaves the statement as its default precedence:
```
 open(FOO || die);
 
  # perl4 opens or dies
  # perl5 opens FOO, dying only if 'FOO' is false, i.e. never
 
```
$: precedence over $:: gone
perl4 gives the special variable, $: precedence, where perl5treats $:: as main package
```
 $a = "x"; print "$::a";
 
  # perl 4 prints: -:a
  # perl 5 prints: x
 
```
Precedence of file test operators documented
perl4 had buggy precedence for the file test operators vis-a-visthe assignment operators. Thus, although the precedence tablefor perl4 leads one to believe -e $foo .= "q" should parse as((-e $foo) .= "q"), it actually parses as (-e ($foo .= "q")).In perl5, the precedence is as documented.
```
 -e $foo .= "q"
 
  # perl4 prints: no output
  # perl5 prints: Can't modify -e in concatenation
 
```
keys, each, values are regular named unary operators
In perl4, keys(), each() and values() were special high-precedence operatorsthat operated on a single hash, but in perl5, they are regular named unaryoperators. As documented, named unary operators have lower precedencethan the arithmetic and concatenation operators + - ., but the perl4variants of these operators actually bind tighter than + - ..Thus, for:
```
 %foo = 1..10;
  print keys %foo - 1
 
  # perl4 prints: 4
  # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
 
```
The perl4 behavior was probably more useful, if less consistent.

General Regular Expression Traps using s///, etc.

All types of RE traps.

s'$lhs'$rhs' interpolates on either side
s'$lhs'$rhs' now does no interpolation on either side. It used tointerpolate $lhs but not $rhs. (And still does not match a literal'$' in string)
```
 $a=1;$b=2;
  $string = '1 2 $a $b';
  $string =~ s'$a'$b';
  print $string,"\n";
 
  # perl4 prints: $b 2 $a $b
  # perl5 prints: 1 2 $a $b
 
```
m//g attaches its state to the searched string
m//g now attaches its state to the searched string rather than theregular expression. (Once the scope of a block is left for the sub, thestate of the searched string is lost)
```
 $_ = "ababab";
  while(m/ab/g){
  &doit("blah");
  }
  sub doit{local($_) = shift; print "Got $_ "}
 
  # perl4 prints: Got blah Got blah Got blah Got blah
  # perl5 prints: infinite loop blah...
 
```
m//o used within an anonymous sub
Currently, if you use the m//o qualifier on a regular expressionwithin an anonymous sub, all closures generated from that anonymoussub will use the regular expression as it was compiled when it was usedthe very first time in any such closure. For instance, if you say
```
 sub build_match {
  my($left,$right) = @_;
  return sub { $_[0] =~ /$left stuff $right/o; };
  }
  $good = build_match('foo','bar');
  $bad = build_match('baz','blarch');
  print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
  print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
  print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";
 
```
For most builds of Perl5, this will print:oknot oknot ok
build_match() will always return a sub which matches the contents of$left and $right as they were the first time that build_match()was called, not as they are in the current call.
$+ isn't set to whole match
If no parentheses are used in a match, Perl4 sets $+ tothe whole match, just like $&. Perl5 does not.
```
 "abcdef" =~ /b.*e/;
  print "\$+ = $+\n";
 
  # perl4 prints: bcde
  # perl5 prints:
 
```
Substitution now returns null string if it fails
substitution now returns the null string if it fails
```
 $string = "test";
  $value = ($string =~ s/foo//);
  print $value, "\n";
 
  # perl4 prints: 0
  # perl5 prints:
 
```
Also see Numerical Traps for another example of this new feature.

s`lhs`rhs` is now a normal substitution

s`lhs`rhs` (using backticks) is now a normal substitution, with nobacktick expansion

 $string = "";
  $string =~ s`^`hostname`;
  print $string, "\n";
 
  # perl4 prints: <the local hostname>
  # perl5 prints: hostname

Stricter parsing of variables in regular expressions
Stricter parsing of variables used in regular expressions
```
 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
 
  # perl4: compiles w/o error
  # perl5: with Scalar found where operator expected ..., near "$opt$plus"
 
```
an added component of this example, apparently from the same script, isthe actual value of the s'd string after the substitution.[$opt] is a character class in perl4 and an array subscript in perl5
```
 $grpc = 'a';
  $opt  = 'r';
  $_ = 'bar';
  s/^([^$grpc]*$grpc[$opt]?)/foo/;
  print;
 
  # perl4 prints: foo
  # perl5 prints: foobar
 
```

m?x? matches only once

Under perl5, m?x? matches only once, like ?x?. Under perl4, it matchedrepeatedly, like /x/ or m!x!.

 $test = "once";
  sub match { $test =~ m?once?; }
  &match();
  if( &match() ) {
  # m?x? matches more then once
  print "perl4\n";
  } else {
  # m?x? matches only once
  print "perl5\n";
  }
 
  # perl4 prints: perl4
  # perl5 prints: perl5

Failed matches don't reset the match variables
Unlike in Ruby, failed matches in Perl do not reset the match variables($1, $2, ..., $`, ...).

Subroutine, Signal, Sorting Traps

The general group of Perl4-to-Perl5 traps having to do withSignals, Sorting, and their related subroutines, as well asgeneral subroutine traps. Includes some OS-Specific traps.

Barewords that used to look like strings look like subroutine calls

Barewords that used to look like strings to Perl will now look like subroutinecalls if a subroutine by that name is defined before the compiler sees them.

 sub SeeYa { warn"Hasta la vista, baby!" }
  $SIG{'TERM'} = SeeYa;
  print "SIGTERM is now $SIG{'TERM'}\n";
 
  # perl4 prints: SIGTERM is now main'SeeYa
  # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")

Use -w to catch this one

Reverse is no longer allowed as the name of a sort subroutine

reverse is no longer allowed as the name of a sort subroutine.

 sub reverse{ print "yup "; $a <=> $b }
  print sort reverse (2,1,3);
 
  # perl4 prints: yup yup 123
  # perl5 prints: 123
  # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()

warn() won't let you specify a filehandle.
Although it _always_ printed to STDERR, warn() would let you specify afilehandle in perl4. With perl5 it does not.
```
 warn STDERR "Foo!";
 
  # perl4 prints: Foo!
  # perl5 prints: String found where operator expected
 
```

OS Traps

SysV resets signal handler correctly
Under HPUX, and some other SysV OSes, one had to reset any signal handler,within the signal handler function, each time a signal was handled withperl4. With perl5, the reset is now done correctly. Any code relyingon the handler _not_ being reset will have to be reworked.
Since version 5.002, Perl uses sigaction() under SysV.
```
 sub gotit {
  print "Got @_... ";
  }
  $SIG{'INT'} = 'gotit';
 
  $| = 1;
  $pid = fork;
  if ($pid) {
  kill('INT', $pid);
  sleep(1);
  kill('INT', $pid);
  } else {
  while (1) {sleep(10);}
  }
 
  # perl4 (HPUX) prints: Got INT...
  # perl5 (HPUX) prints: Got INT... Got INT...
 
```

SysV seek() appends correctly

Under SysV OSes, seek() on a file opened to append >> now doesthe right thing w.r.t. the fopen() manpage. e.g., - When a file is openedfor append, it is impossible to overwrite information already inthe file.

 open(TEST,">>seek.test");
  $start = tell TEST;
  foreach(1 .. 9){
  print TEST "$_ ";
  }
  $end = tell TEST;
  seek(TEST,$start,0);
  print TEST "18 characters here";
 
  # perl4 (solaris) seek.test has: 18 characters here
  # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here

Interpolation Traps

Perl4-to-Perl5 traps having to do with how things get interpolatedwithin certain expressions, statements, contexts, or whatever.

@ always interpolates an array in double-quotish strings

@ now always interpolates an array in double-quotish strings.

 print "To: [email protected]\n";
 
  # perl4 prints: To:[email protected]
  # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
  # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string

Double-quoted strings may no longer end with an unescaped $
Double-quoted strings may no longer end with an unescaped $.
```
 $foo = "foo$";
  print "foo is $foo\n";
 
  # perl4 prints: foo is foo$
  # perl5 errors: Final $ should be \$ or $name
 
```
Note: perl5 DOES NOT error on the terminating @ in $bar
Arbitrary expressions are evaluated inside braces within double quotes
Perl now sometimes evaluates arbitrary expressions inside braces that occurwithin double quotes (usually when the opening brace is preceded by $or @).
```
 @www = "buz";
  $foo = "foo";
  $bar = "bar";
  sub foo { return "bar" };
  print "|@{w.w.w}|${main'foo}|";
 
  # perl4 prints: |@{w.w.w}|foo|
  # perl5 prints: |buz|bar|
 
```
Note that you can use strict; to ward off such trappiness under perl5.
$$x now tries to dereference $x
The construct "this is $$x" used to interpolate the pid at that point, butnow tries to dereference $x. $$ by itself still works fine, however.
```
 $s = "a reference";
  $x = *s;
  print "this is $$x\n";
 
  # perl4 prints: this is XXXx   (XXX is the current pid)
  # perl5 prints: this is a reference
 
```

Creation of hashes on the fly with eval "EXPR" requires protection

Creation of hashes on the fly with eval "EXPR" now requires either both$'s to be protected in the specification of the hash name, or both curliesto be protected. If both curlies are protected, the result will be compatiblewith perl4 and perl5. This is a very common practice, and should be changedto use the block form of eval{} if possible.

 $hashname = "foobar";
  $key = "baz";
  $value = 1234;
  eval "\$$hashname{'$key'} = q|$value|";
  (defined($foobar{'baz'})) ?  (print "Yup") : (print "Nope");
 
  # perl4 prints: Yup
  # perl5 prints: Nope

Changing

 eval "\$$hashname{'$key'} = q|$value|";

 eval "\$\$hashname{'$key'} = q|$value|";

causes the following result:

 # perl4 prints: Nope
  # perl5 prints: Yup

or, changing to

 eval "\$$hashname\{'$key'\} = q|$value|";

causes the following result:

 # perl4 prints: Yup
  # perl5 prints: Yup
  # and is compatible for both versions

Bugs in earlier perl versions

perl4 programs which unconsciously rely on the bugs in earlier perl versions.

 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
 
  # perl4 prints: This is not perl5
  # perl5 prints: This is perl5

Array and hash brackets during interpolation
You also have to be careful about array and hash brackets duringinterpolation.
```
 print "$foo["
 
  perl 4 prints: [
  perl 5 prints: syntax error
 
  print "$foo{"
 
  perl 4 prints: {
  perl 5 prints: syntax error
 
```
Perl 5 is expecting to find an index or key name following the respectivebrackets, as well as an ending bracket of the appropriate type. In orderto mimic the behavior of Perl 4, you must escape the bracket like so.
```
 print "$foo\[";
  print "$foo\{";
 
```
Interpolation of \$$foo{bar}
Similarly, watch out for: \$$foo{bar}
```
 $foo = "baz";
  print "\$$foo{bar}\n";
 
  # perl4 prints: $baz{bar}
  # perl5 prints: $
 
```
Perl 5 is looking for $foo{bar} which doesn't exist, but perl 4 ishappy just to expand $foo to "baz" by itself. Watch out for thisespecially in eval's.

qq() string passed to eval will not find string terminator

qq() string passed to eval

 eval qq(
  foreach \$y (keys %\$x\) {
  \$count++;
  }
  );
 
  # perl4 runs this ok
  # perl5 prints: Can't find string terminator ")"

DBM Traps

General DBM traps.

Perl5 must have been linked with same dbm/ndbm as the default for dbmopen()
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)may cause the same script, run under perl5, to fail. The build of perl5must have been linked with the same dbm/ndbm as the default for dbmopen()to function properly without tie'ing to an extension dbm implementation.
```
 dbmopen (%dbm, "file", undef);
  print "ok\n";
 
  # perl4 prints: ok
  # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
 
```

DBM exceeding limit on the key/value size will cause perl5 to exit immediately

Existing dbm databases created under perl4 (or any other dbm/ndbm tool)may cause the same script, run under perl5, to fail. The error generatedwhen exceeding the limit on the key/value size will cause perl5 to exitimmediately.

 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
  $DB{'trap'} = "x" x 1024;  # value too large for most dbm/ndbm
  print "YUP\n";
 
  # perl4 prints:
  dbm store returned -1, errno 28, key "trap" at - line 3.
  YUP
 
  # perl5 prints:
  dbm store returned -1, errno 28, key "trap" at - line 3.

Unclassified Traps

Everything else.

require/do trap using returned value
If the file doit.pl has:
```
 sub foo {
  $rc = do "./do.pl";
  return 8;
  }
  print &foo, "\n";
 
```
And the do.pl file has the following single line:
```
 return 3;
 
```
Running doit.pl gives the following:
```
 # perl 4 prints: 3 (aborts the subroutine early)
  # perl 5 prints: 8
 
```
Same behavior if you replace do with require.
split on empty string with LIMIT specified
```
 $string = '';
  @list = split(/foo/, $string, 2)
 
```
Perl4 returns a one element list containing the empty string but Perl5returns an empty list.

As always, if any of these are ever officially declared as bugs,they'll be fixed and removed.

Source : perldoc.perl.org - Official documentation for the Perl programming language
Site maintained by Jon Allen (JJ) See the project page for more details
Documentation maintained by the Perl 5 Porters

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