Using ideas previously mentioned, and throwing in a few of my own, this can be brought down to 98 characters:

sub decipher {
@l=(_,a..z,'.');@n{@l}=0..27;@c=@n{split//,pop};$"=my$i;$p[$_[0]*$i++%
+@c]=$l[($_+$i)%28]for@c;"@p"
}
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