This code should do what you need:

open(F, '<', './files.txt') or die "open: $!";
while ($filename = <F>) {
  chomp($filename);
  $dot = rindex($filename, '.');
  if ($dot > -1) {
    $extension = substr($filename, $dot+1);
    $extensions{$extension}++;
  } else {
    $extensions{'_without_extension'}++;
  }
}
close(F);

while (($key, $value) =  each %extensions) {
  print "$key -> $value\n";
}
[download]

Ciao, Valerio

update: thanks liz!