Your descriptions are so jumbled and lacking in detail it is impossible to reach any conclusions based upon them.

Your original post suggested that do "didn't work" when used in a thread. This is easily demonstrated to be untrue:

c:\test>copy con test.pl
print 'Hello from test.pl';
^Z
        1 file(s) copied.

c:\test>perl -Mthreads -E"async{ do 'test.pl' }->join"
Hello from test.pl
c:\test>
[download]

Now you say that is now working for you but "something else isn't working". That isn't much to go on!

If your program is so large and has so many dependencies that you cannot post it here, then you need to produce a small, self contained example (like the one I just posted above), that demonstrates the problem you are having. Because your wordy descriptions mean absolutely nothing to me.

The chances are, if you go through the process of writing a stand alone demo, you will probably realise your error yourself, but if you don't, the post it here and we'll be able to help you.