open(DIR,"$base_dir\\$dir") or die "$dir failed to open: $!";
while (my $file = readdir(DIR)) {
   next unless $file =~ /\.txt$/;
   # etc, etc.
   open(IN,"$full_name") || die "can't open $!";
   while (my $line = <IN>) {
      # processing
   }
   close(IN);
}
closedir(DIR);