#!/usr/bin/perl

use strict;
no warnings;

use Scalar::Util qw/ dualvar /;

my $foo = dualvar 1, "foo";
my $bar = dualvar 2, "foo";

print "Using dualvar$/";
print '$foo', ($foo eq $bar ? " eq " : " ne "), '$bar', $/;
print '$foo', ($foo == $bar ? " == " : " != "), '$bar', $/;

__END__
Using dualvar
$foo eq $bar
$foo != $bar
[download]

Using overload could result in similarly confusing comparisons. This isn't as contrived as it sounds:

#!/usr/bin/perl

use strict;
use warnings;

use URI;

my $foo = URI->new('http://www.perlmonks.org');
my $bar = URI->new('http://www.perlmonks.org');

print "Using URI$/";
print '$foo', ($foo eq $bar ? " eq " : " ne "), '$bar', $/;
print '$foo', ($foo == $bar ? " == " : " != "), '$bar', $/;

__END__
Using URI
$foo eq $bar
$foo != $bar
[download]

URI defines an overload for stringification, but not for numification. In numerical context, objects evaluate to their location in memory. And the two different objects are stored in different locations, so == returns false.

So be careful with == and eq :)