Hi,
I'd write the whole thing as:

@letter = qw/a b c w x y/;

foreach $x (@letter) {
  $m = 0;
  open(DATA,"data.txt") or die
    "couldn't open data\n$!";
  while (<DATA>) {
    $m += ($_ =~ s/$x/$x/g);
  }
  close(DATA);
  print "count $x = $m\n";  
}
[download]

(note the use of foreach, or and $! ...)

Regards...

Stefan