thconv.pl

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

sub thconv
{
    my $n = shift;
    my ($offset, $divider, $digits) = (1, 0, 0);
    do {
        $offset  *= 0x100;
        $divider += $offset;
        $digits  += 2;
    } while ($n >= $divider);
    return sprintf("%0*X", $digits, ($n + $offset) % $divider);
}

{
    while (<>) {
        chomp;
        my $n = hex($_);
        printf("%X => %s\n", $n, thconv($n));
    }
}