#!/usr/bin/perl -w
use strict;

my $line='boy boy girl boy girl boy girl girl';

my $count =()= $line =~ /boy/g;      #number of boys on line
print "number of boys = $count\n";

my $num = int(rand($count)) + 1;     #1..$count
print "looking for nth boy: $num\n";

my $found_position =-1;
while ($num--)
{
   $found_position= index($line,"boy", $found_position+1);
   print "replace index = $found_position\n";
}

substr ($line, $found_position, length("boy"), "cowboy");

print "$line\n";

__END__
One example run:
number of boys = 4
looking for nth boy: 3
replace index = 0
replace index = 4
replace index = 13
boy boy girl cowboy girl boy girl girl