use strict;

opendir(INDIR,"/path/to/directory/with/files") or die $!;

while (my $fname = readdir(INDIR)){
      next unless $fname =~ m@\.txt$@;
      open FIN," < $fname" or die "$fname: $!";
      my @stuff=<FIN>;
      close FIN;

      #process lines....
}
[download]

There's another way of doing it.