Unfortunately, this one isn't very random - at least with regard to the first occurrence of 'boy'. I see where you tried to correct it with the '- 1', but that doesn't work:

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

my %data;
for (1..10000){
    my $str = "boy boy girl boy girl boy girl girl";
    my $l = int(rand(rindex($str,"boy") - 1));
    $str =~ s/^(.{$l}.*?)\bboy\b/$1man/;
    $data{$str}++;
}

print "$_: $data{$_}" for sort keys %data;
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Output:

boy boy girl boy girl man girl girl: 3323
boy boy girl man girl boy girl girl: 4310
boy man girl boy girl boy girl girl: 1881
man boy girl boy girl boy girl girl: 486
[download]

(Don't ask me why I know about this problem. :)))