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#!/usr/local/bin/perl -w
use strict;
...
sub getn_split {
    return +(split //, $_[0])[$_[1]-1];
}

Benchmark: timing 100000 iterations of split, substr, unpack...
     split: 20 secs (19.39 usr  0.00 sys = 19.39 cpu)
    substr:  0 secs ( 1.78 usr  0.00 sys =  1.78 cpu)
    unpack:  2 secs ( 2.89 usr  0.00 sys =  2.89 cpu)