With current version in Debian stretch/sid (4.07), and current git master, when doing the following, I get that uninitialized warning, and the empty "filename":
$ echo | pod2man -nTest
Use of uninitialized value in concatenation (.) or string at /usr/bin/pod2man line 73.
pod2man: unable to format
With current version in Debian stretch/sid (4.07), and current git master, when doing the following, I get that uninitialized warning, and the empty "filename":