Another way...
I used "cowboy" instead of "man" for the substitute for "boy" to make sure that I could put a different length string in there. This will have to "get smarter" if for example "girlyboy" was one of tokens in $line".

#!/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
[download]

Update: Another solution:

#!/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
[download]

oh, "Girly Boy" came about from an off hand comment from our out-going California Governor, Arnold Schwarzenegger - it became a bit of a joke, nothing was meant in any seriously derogatory way.

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