#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my $line='boy boy girl boy girl boy girlyboy girl girl';
my @indicies;

while ($line =~ /\bboy\b/gi)
{
   push (@indicies, pos($line)-3); #pos is a bit tricky
}

print Dumper \@indicies;

my $num = int(rand(@indicies));     #0..$count
print "line = $line\n";
print "looking for nth boy: ",$num+1,"\n";

substr ($line, $indicies[$num], length("boy"), "cowboy");

print "       $line\n";

__END__
Some runs:
$VAR1 = [
          0,
          4,
          13,
          22
        ];
line = boy boy girl boy girl boy girlyboy girl girl
looking for nth boy: 3
       boy boy girl cowboy girl boy girlyboy girl girl
.....
$VAR1 = [
          0,
          4,
          13,
          22
        ];
line = boy boy girl boy girl boy girlyboy girl girl
looking for nth boy: 1
       cowboy boy girl boy girl boy girlyboy girl girl
.....
$VAR1 = [
          0,
          4,
          13,
          22
        ];
line = boy boy girl boy girl boy girlyboy girl girl
looking for nth boy: 4
       boy boy girl boy girl cowboy girlyboy girl girl