This is very strange.

If you change that line to the following (removing $msg)::

print STDERR $pkg . " (" . $$ . "): ";

...it begins to work properly.

Before that line if you add:

die $msg;

....it also hangs.

However if you add:

die $msg;

...before:

# Remove windows linefeeds (breaks unix terminals).
$msg =~ s/\r//g unless ($^O eq 'MSWin32');

...then it works.

This must be something to do with the contents of $msg.