opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@files = grep {-f "$some_dir/$_" } readdir(DIR);
closedir DIR;

##</code><code>##

foreach (@files) {
   /^(\w+)\W+(\w+)\W+(\w+)\W+(\w+)$/;
   next unless $1 && $2 && $3 && $4;
   rename "$some_dir/$_", "$some_dir/$1 $2 - $3 $4.mp3"
      or die "Could not rename file: $!";
}