The main package doesn't call anything in package mutex. There of course are issues in that package mutex is contained with the file package.pl and there is nothing to say how main:: would even find the sub "update" which is contained in the package mutex which itself is contained within the file package.pl

I think some more work is in order in order to get a submission that actually runs. I don't see how even with Perl 4 that this code would work or that the the existence or not of file: package.pl would even matter at all.

Update:
I started working with Perl in the 1990's with Perl 5.6. As Perl has evolved, I've seen some of my old code at least produce warnings with new versions of Perl that it didn't before and I've had to modify that code. Perl 4 is very old. It could be time to just re-write it?