A nit - don't sort the arrays until you've done your @k1 <=> @k2 test. If that returns true then you wasted two sort() ops. Also, do your cmp operation once and stash the result in a my() variable. Its fast, easy and saves redundant work.

sub compare_hash_keys {
    my ($h1,$h2) = @_;
    my $test; # A temporary variable for holding a result so it doesn'
+t have to be recomputed.

    my @k1 = keys %$h1;
    my @k2 = keys %$h2;

    $test = @k1 <=> @k2 and return $test;

    for ( 0 .. $#k1 ) {
        $test = $k1[$_] cmp $k2[$_] and return $test;
    }

    return;
}
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