perl: fix a regression with caller() not indicating the starting *.psgi program

In 2.0.1-41-g3480c30 I introduced a regression with how the top-level
stackframe would appear within Perl programs. Before we'd show the
filename of the *.psgi file, but after we just showed "-e".

We can retain the bugfix I added in 2.0.1-41-g3480c30 while having a
sensible stacktrace by overriding the file via the #line directive.
Merge pull request #565 from avar/avar/fix-perl-stacktrace

perl: fix a regression with caller() not indicating the starting *.psgi ...