#!/usr/bin/perl
use warnings;
use strict;
use feature 'say';

sub replace {
    my ($string, $from, $to, $count) = @_;
    my $pos = '0E0'; # plain 0 means the string begins with $from
    while ($count-- and $pos >= 0) {
        $pos = index $string, $from, $pos eq '0E0' ? $pos : $pos + 1;
    }
    substr $string, $pos, 1, $to if $pos > 0;
    return $string;
}

say replace($_, ',' => '|', 2) for qw( a,b,c,d
                                       pq,rs,tu,vw
                                       ,s,t,a,r,t
                                       ,,yuck
                                       1,2
                                     );