To reply to my own reply, this will also be a problem:

my $username; my $passwd;
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Effectively, that makes it so that every time conv_func is called, $username and $passwd are blank - probably not what you want. The simplest solution is to add the my declarations to the top of the program and then never, ever, ever redeclare them :) Get rid of the line I quoted above from conv_func and get rid of the my keyword in front of $passwd and $username in the if block.