opendir DIR, $dir or die "Failed to open directory $dir: $!";
foreach my $file (readdir DIR)
{
                    # much the same as using fileparse
    my ($number, $ext) = split /\./, basename($file);
    next unless $ext eq "txt";
    next unless $number =~ /^\d+$/;
    next unless $number < $start || $number > $end;

    push @ARGV, "$dir/$file";
}