You can use the modulus operator for that.

my $is_even = $number % 2 == 0;
my $is_odd  = $number % 2 == 1;
[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)