Here's an alternative way that uses three idioms from the Perl FAQ.

How can I count the number of occurrences of a substring within a string?
How do I get a random number between X and Y?
How do I change the Nth occurrence of something?

#!perl

use strict;
use warnings;
use feature qw( say );
use English;

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

say $line;

my $match_pattern      = qr/\bboy\b/;
my $replacement_string = 'man';

my $total_matches = () = $line =~ m/$match_pattern/g;

# Generate a random number from 1 to $total_matches...
my $random_number = int(rand($total_matches)) + 1;
my $match_number  = 1;

$line =~ s{
    $match_pattern
}{
    $match_number++ == $random_number ? $replacement_string : $MATCH;
}gex;

say $line;

exit 0;
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