#!/usr/bin/perl -w

use strict;
use warnings;
use Data::Dumper;

my $x;    # $x is undef
my $y;    # $y is undef too

my %hash;
$hash{$x} = 123;

# This will throw warnings, but both $hash{$x} and $hash{$y} are 123
printf "\$hash{\$x} = %s; \$hash{\$y} = %s\n", $hash{$x}, $hash{$y};

print "Keys of \$hash => %s\n", Dumper([keys %hash]);

__END__

Output:

Use of uninitialized value in hash element at x line 11.
Use of uninitialized value in hash element at x line 11.
Use of uninitialized value in hash element at x line 14.
Use of uninitialized value in hash element at x line 14.
$hash{$x} = 123; $hash{$y} = 123
Keys of $hash => $VAR1 = [
          ''
        ];