It looks to me as if undef used as a hash key gets turned into an empty string. 

use Test::More tests => 6;

my %h = ( undef() => 1 );

foreach my $key ( keys %h ) {
    ok( defined $key, 'key is defined' );
    is( $key, '', 'key is empty string' );
}
is_deeply( [ '' ], [ keys %h ], 'undef key' );
is_deeply( [ 1 ], [ values %h ], 'values works' );
is( $h{undef()}, 1, 'undef key accesses value' );
is( $h{''},      1, 'empty string key accesses value' );

[download]

Even when I try to be rude and slip it in there, it won't go. 

package UndefKey;
sub TIEHASH { bless [], __PACKAGE__ }
sub FIRSTKEY { undef }
sub NEXTKEY { undef }
sub FETCH { 1 }

package main;
#my %h = ( undef() => 1 );
tie my %h, 'UndefKey';

[download]

(Running the same tests as before, only the last two succeed, and the ones in the foreach do not run because keys returns an empty list.)

In reply to Re: undef as a key in a hash. by kyle
in thread undef as a key in a hash. by fenLisesi

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