Here's a non-obfuscated version:

use strict;
use warnings;
use 5.010;

use List::Util qw{ shuffle };

my @numbers = (1 .. 100_000);
my @rand_numbers = shuffle @numbers;


my @fnames = qw{ joe_IMG_27445.JPG 
                 Mary_34555.jpg };

for my $fname (@fnames) {
    
    my $rand_number = shift @rand_numbers;

    my @pieces = split /_/, $fname, 2;
    my $new_fname = "$pieces[0]_$rand_number.jpg";

    say $new_fname;
}

--output:--
joe_18119.jpg
Mary_46301.jpg
[download]