Sometimes code involving IO handles can be confusing – that is because IO handles are of many kinds in Perl, like globs (the old way), glob refs, IO handles in scalar variables (produced with open and other IO operations), and legit objects which derive from IO::Handle, IO::File, etc.

The code in Email::Sender::Transport::Print wants an IO::Handle, or something that satisfies

This is far from ideal – something more TIMTOWTDI, would be a check like the one of Scalar::Util::openhandle()

As you discovered, $fh is a glob ref, and does not satisfy this precondition. But that does

This fact is kind of sparse in perl documentation. For example, you can read that suggested at item 7 at Making References

You may use it in your code right now and / or submit a patch to Email::Sender::Transport::Print maintainer.