You're none too clear about what you're trying to accomplish. The keys of a hash are all unique. You can check existence of keys from an array with grep over a slice,

my %found_keys;
for my $key (keys %hoa) {
    $found_keys{$key} = [grep {
        exists $hoa{$_} and $_ ne $key
    } @{$hoa{$key}}];
}
[download]

That gives you another HoA containing only the elements of the original which match other internal keys than their own.

I'm not positive that's what you want, but I hope it helps.