You don't have any captures before your use of $^N, so it's always undef when you use it. Change 

(?><+)

[download]

to 

((?><+))

[download]

By the way, 

use vars qw' $LEN ';

[download]
would be better as 
local our $LEN;

[download]

In reply to Re: experimental regex (?(?{code}) conditional (??{ 'code' }) ) by ikegami
in thread experimental regex (?(?{code}) conditional (??{ 'code' }) ) by Anonymous Monk

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