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

my $line='boy boy girl boy girl boy girl girl';
my ($pos, @pos);
my $last = 0;
{
    $pos = index $line, 'boy', $last;
    last if $pos == -1;
    push @pos, $pos;
    $last = $pos + 1;
    redo;
}
substr $line, $pos[rand(@pos)], 3, 'man';
print "$line\n";