my $files = 0;
for my $file ("sfull1ns.dat","sfull2ns.dat","sfull3ns.dat")  {
  open (my $fh,'<',$file) or die "Can’t open file $file: $!";
  while (my $line = <$fh>) {
  ...
  }
  $files++
  close $fh or die "Can’t close file $file: $!";
}