A nice way of adding incremental roman numerals, as you do at the end of your script, would be to use the Math::Roman module (handily mentioned in the Perl Cookbook). This would impose no arbitrary limit on the messages you receive from the same person - it even goes above the 'highest' Roman numeral of 5000.

#!/usr/bin/perl -w
use strict;

use Math::Roman qw(roman);
my $roman = new Math::Roman;

$\ = "\n";

while (<DATA>) {
    chomp;
    my $old_name = $_;

    if ($old_name =~ /^(.+\d{6})(\w+)?(\.htm)$/) {
        my $old_roman = $2 || 1; 
        my $new_roman = roman("$old_roman") + 1;
        my $new_name = $1 . $new_roman . $3;
        print "$old_name => $new_name";
    }
}

__DATA__
filename230804.htm
filename230804IV.htm
filename230804V.htm
filename230804II.htm
filename230804X.htm
[download]

Output:

filename230804.htm => filename230804II.htm
filename230804IV.htm => filename230804V.htm
filename230804V.htm => filename230804VI.htm
filename230804II.htm => filename230804III.htm
filename230804X.htm => filename230804XI.htm
[download]

Of course, you could simply append 'normal' numbers to achieve the same result, but that wouldn't be half as much fun...