Try a segexer (reverse regex)

my $str = 'This_is_a_song_.mp3';
$str = reverse $str;
$str =~ s/_//;
$str = reverse $str;
[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)