i'm not sure what the problem is. i tried your code above for initializing %H and $H{y} was never 'z'. the following code also works for me:

my $undef_var = undef;

my %hash;
@hash{foo, bar, baz} = (1, $undef_var, 2);

for (sort keys %hash)
{
        printf "%s => %s\n", $_, defined $hash{$_} ? $hash{$_} : 'unde
+f';
}
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