Don't use global variables:

print(
     test_reg('fudge','fudge') ? "matches\n" : "no matches\n"
);

sub test_reg {
    my $this = shift;
    my $that = shift;

    return $this =~ /$that/i;
}
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)