Here's mine. Not particularly interesting, really, and probably missing out on a few standard golf tricks. 
sub c {my(@a,%h,$i)=split//,pop;my$s=pop;$s=~s!(.)!$h{$1}||=$a[$i++]!g
+e;$s}

[download]
67 chars.

Update: got rid of one character by using substr: 

sub c {my($a,%h,$i)=pop;my$s=pop;$s=~s!(.)!$h{$1}||=substr$a,$i++,1!ge
+;$s}

[download]
66 chars.

Update 2: if you don't need strict: 

sub c {($s,$a)=@_;my(%h,$i);$s=~s!(.)!$h{$1}||=substr$a,$i++,1!ge;$s}

[download]
61.

Update 3: Oh, and the reason I'm still initializing %h and $i is so that the sub will work on repeated invocations. If we don't *have* to do that, this will do: 

sub c {($s,$a)=@_;$s=~s!(.)!$h{$1}||=substr$a,$i++,1!ge;$s}

[download]
51.

Update 4: Use $_: 

sub c {($_,$a)=@_;s!(.)!$h{$1}||=substr$a,$i++,1!ge;$_}

[download]
47.

Update 5: Okay, so none of these actually *work* per the spec. Thanks, tilly. :)

In reply to Re: (Golf) Cryptographer's Tool #1 by btrott
in thread (Golf) Cryptographer's Tool #1 by Masem

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