#!/usr/bin/env perl -l

use strict;
use warnings;
use autodie;

my $highest = 0;
my $re = qr{pm_1069813_results(\d+)\.txt};
opendir my $dh, '.';
map { /$re/ and $1 > $highest and $highest = $1 } readdir $dh;
closedir $dh;
my $next_file = 'pm_1069813_results' . ++$highest . '.txt';

print "Next file: $next_file";

##</code><code>##

Next file: pm_1069813_results4.txt

##</code><code>##

$ ls -1 pm_1069813_results*.txt
pm_1069813_results1.txt
pm_1069813_results2.txt
pm_1069813_results3.txt