Looking at your code it seems you already know how to pick up array elements. Do you know how to pick up hash elements too? I guess so, then it is fundamentally trivial to perform the remaining test.

As a side note, if you don't already know it, grep is your friend. For example, assuming you do not really want to store the differing keys as "$_ ", then this should be equivalent to your code:

@diff=grep { !exists $old_gp{$_} } keys %new_gp;
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And in case you do want to store them that way, then you can chain with a map.