Here are mine,

Update : since I didn't understand anyone elses (give or take) I've added explanations to mine

The first is obvious (51 chars)
The second is stated in a couple different ways (43 chars, 50 chars obfu'd)

sub obvious{
#  (explained : count from zero until Z, return the current count if w
+e found the position we are looking for)
    #        1         2         3         4         5         6
    #23456789012345678901234567890123456789012345678901234567890
    $n=pop;for(;$n ne((0..9),(A..Z))[$a];$a++){}"$a"||0;
}
[download]

This one is more fun, it uses bitwise arithmatic to get the solution. The commented line is the same as the first, but more obfu

sub fun {
# (explained : reduce the numbers to their base2 (binary)
# representation.  we always care about the
# lowest four bits ($n & 0x1111 is $n & 15).  If the ord of
# the number is in the A-Z range the 7th bit will be set
# ($n >> 6) will be true.  In this case, also include the
# 6th bit in the number ($n & 31) instead of ($n & 15).
# Since A-Z starts at ten, add 9 as well)
    #        1         2         3         4         5         6
    #23456789012345678901234567890123456789012345678901234567890
    $n=ord(pop);$a=($n>>6)?(($n&31)+9):($n&15);
    # same thing, but obfu
  # $n=ord(pop);$a=($n&(1<<(4+($n>>6)))-1)+($n>>6&&9);
}
[download]

Enjoy, or not, at your leisure.

-jackdied

PS, posting with mozilla is the biggest pain in the ass ever. It tries to do smart things with wrapping in a text box, and translating spaces and returns for you with some AI that makes posting code hell.