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my @diff = grep { !exists $old_gp{$_} } keys %new_gp;
push @diff, grep { !exists $new_gp{$_} } keys %old_gp;

my (%foo, %bar);
# Populate %foo and %bar
...
   if (exists $bar{$_} && $bar{$_} eq $foo{$_}) { $_ }
   else { () }
} keys %foo;

my (@only_in_foo, @kv_different, @kv_equal);
foreach (keys %foo) {
...
   }
}
my @only_in_bar = grep { ! exists $foo{$_} } keys %bar;