The problem is related to the use of memory files. Apparently Perl issues the uninitialized warning upon the second time (and third etc) a memory file is created.

#!/usr/bin/perl -w

my $output;
print "first\n";
open (FH, '>', \$output);
print FH "Hello world\n";
close(FH);
$output=undef;
print "second\n";
open (FH, '>', \$output);
print FH "Hello world\n";
close(FH);
[download]

Output:

first
second
Use of uninitialized value in open at o4.pl line 10.
[download]

Seems to me like an overzealous action by Perl's warning system but nothing alarming so far.
Like said by the other monks you can set $output to the empty string as a workaround to avoid the warning alltogether.