@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";  
}