When you say "3rd array slice", you really mean "array element whose index is 3". Be careful with that terminology because "array slice" has a different, and quite specific, meaning in Perl: see perldata - Slices.

There was a thread that started yesterday that involved ranges: Perl hit counter for multiple ranges, optimization or options in other languages?. It's not exactly what you're doing but you may pick up a few tips or ideas from it.