Changing the $num variable to a package variable fixes the problem for me as well. Here's what seems to be happening:

• The first call to bin_to_dec compiles the regular expression. The (?{code}) block is compiled too, and the name $num is bound to the location of the lexical variable $num within the sub's scope
• Any subsequent call to the bin_to_dec sub creates a new $num variable because of the my statement. But the regular expression is the same one from before (not recompiled), so the name $num is still bound to the lexical from the previous pad!

You can verify this by printing $num from within the regular expression. It keeps getting updated correctly, but it's not the same $num that the return statement sees. Anyway, making it a package variable with local instead of my clearly fixes it.

Also, a minor nit: An easier way to read in binary numbers left-to-right is:

m/ (?{ $num = 0; }) (?: ([10]) (?{ $num = 2*$num + $^N; }) )+ /x
[download]

i.e, shift left (which in binary multiplies by two), and carry in the new bit. This extends to any base representation (just exchange 2 for the base). You never have to know the length in advance. As a former TA for a models of computation class, I couldn't let that one slide ;)

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