#!/usr/bin/perl
use strict;
use warnings;

my $topdir = 'D:/PGN/ELSEVIER/FNS/1(1)/press';
opendir HU, $topdir; # don't quote a single scalar
my @files = grep /\.txt$/i, readdir HU;   # will also match .TXT
closedir HU;

my $biggest = 0;  #keep track of largest number found
for my $frame (@files){
    if($frame =~ m|^partial_pressps_report(\d+)|){
        my $n = $1;            # capture number from filename
        if( $n > $biggest ){   # is it bigger than $biggest?
            $biggest = $n;     # then save it
        }
    }
}
$biggest++;      # got the largest one, now increment it
print $biggest;  # and print it