use strict;
use warnings;

my ($dir, $file) = './';  ### Directory to process
opendir(DIR, $dir) || die;
while ($file = readdir(DIR)) {
    if ($file !~ /_my_guitar\.mp3/ && $file =~ /(.*)\.mp3/) {
        $_ = "$1_my_guitar.mp3";
        `mv $dir$file $dir$_`;    ### Or some other rename
    }
}