== numifies its arguments. Non-numeric strings numify as 0 so all such strings equal each other and equal 0 when using ==. Consider:

use strict;
#use warnings;

sub match {
  my ($lhs, $rhs) = @_;
  print "$lhs == $rhs\n" if $lhs == $rhs;
  print "$lhs != $rhs\n" if $lhs != $rhs;
}

match ('0', '1');
match ('0', '0');
match ('1', '1');
match ('0xxx', '1xxx');
match ('0xxx', '0xxx');
match ('0', 'YYY');
match ('xxx', 'yyy');
match ('yyy', 'YYY');

[download]

Prints:

0 != 1
0 == 0
1 == 1
0xxx != 1xxx
0xxx == 0xxx
0 == YYY
xxx == yyy
yyy == YYY

[download]

Note that warnings are not enabled, otherwise you will get a bunch of warnings at runtime.

DWIM is Perl's answer to Gödel

In reply to Re: is behaviour undefined for string comparision with == by GrandFather
in thread is behaviour undefined for string comparision with == by edwardt_tril

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