nice, srawls... here it is down to 51:

sub c{
($_,$a)=@_;s!.!chr($h{$&}||=($a=~/./g,ord$&))!ge;$_
#23456789_123456789_123456789_123456789_123456789_1
}
[download]

It doesn't work with null either, but as i read it, thats fine.

update: oops, here's one that resets %h and is reusable, as per the given example, at 54:

sub c{
($_,$a,%h)=@_;s!.!chr($h{$&}||=($a=~/./g,ord$&))!ge;$_
#23456789_123456789_123456789_123456789_123456789_1234
}
[download]

--sean