Do the parent keys have to match?

Otherwise,

my @unseen = do {
     delete %{$hash1->{$a}{$b}}, keys %{$hash2->{$d}{$e}};
     keys %{$hash1->{$a}{$b}};
}
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That leaves it up to you to set up the parent keys you want in the match. That's destructive of $hash1.

$a and $b are poor choices for variable names. They are sacred to sort and can have side effects.