Cool, this is what I came up with yesterday:

[Dog and Pony]

Oh, I should have known there would be a thread like this one here somewhere. :)

Anyways, here is my best shot, as it were:

map{$r+=36**$i++*(/\D/?-55+ord:$_)}reverse split'';

The result going into $r, this is 50 or 51 characters depending on if you count the semi-colon. Since this was my first attempt at something like this ever, I think it was pretty good.

As proof of concept, I did try some of the other variants aswell (you only replace the '36' above):

#!/usr/bin/perl

# Base 36, [0-9A-Z]
$_="II";
map{$r+=36**$i++*(/\D/?-55+ord:$_)}reverse split'';
print "$r\n";

# Hex
$_="29A";
map{$t+=16**$j++*(/\D/?-55+ord:$_)}reverse split'';
print "$t\n";

# Bin
$_="1010011010";
map{$s+=2**$k++*(/\D/?-55+ord:$_)}reverse split'';
print "$s\n";

# Dec :)
$_="666";
map{$u+=10**$l++*(/\D/?-55+ord:$_)}reverse split'';
print "$u\n";

Comments on how to not beat this one, but rather if this special path could have been improved would be appreciated. I was looking into some of the deprecated stuff about a split in scalar mode going into @_, but although that shortened the routine with one character, it also produced the wrong result by -36, and with that fixed, it had lost again. :) Same thing with all my tries to get rid of that pesky reverse.