use strict;
use warnings;
use String::Random;

my $oldfile = 'picture.jpg';
my $random  =  String::Random->new;

if ($oldfile =~ /\.jpg$/)
{
    my $oldname = $oldfile =~ s/\.jpg$//r;
    my $newfile = $random->randpattern('.' x length $oldname) . '.jpg';
    print "$oldfile will be renamed $newfile\n";
}