my %oldfiles;
    foreach $file (@FileArray){
          if (length($file)>13){
              $shortenedname = substr($file, 0, 8);
              $shortenedname = $shortenedname.".mp3";
          } else {$shortenedname = $file;}
          if ( $oldfiles{$shortenedname}++ ) {
      # short name already exist, try again
...