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' );