I would suspect something like this somewhere in the code:

...
my $mysub = some_op_returning_empty_string;
$mysub->();
[download]

Advice: brute-force it. Stick in some print statements using divide-and-conquer, and look for that kind of sub invocation. Maybe do a "grep '\$.*-> *(' filename" on the code.

-- 
I hate storms, but calms undermine my spirits.
 -- Bernard Moitessier, "The Long Way"