I think that you are thinking of opening a file for input. You won't need a regex for that. For example, here's the simplest example that I could think of to get you started:

#!perl
#xyz.pl

use strict;
use warnings;

my $file = '3-4-111.txt';

open FH, '<', $file or die $!;
my(@lines) = <FH>;
@lines = sort @lines;
foreach my $line(@lines) {
    print $line, "\n";
}
close FH;
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Now, if you need to find all the files in a directory that end in .txt, you could use glob:

#!/usr/bin/perl

use strict;
use warnings;

my @files = glob('/your/dir/*.txt');
foreach my $file(@files) {
    print $file, "\n";
}
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To answer your first question---A regex for finding a file with the extension .txt would simply be: /\.txt$/.