If I understand the question correctly, you'd like to be able to run sub A in file A from file B. To do this, you can use Exporter:

File A

    package PackageName;
    require Exporter;

    our @ISA        = qw(Exporter);
    our @EXPORT     = qw(moo);      # Put function names, seperated by
+ spaces,
                                    # that you want to be able to use
    sub moo {
        # do stuff
    }
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File B

    use PackageName;
    moo();
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Basically, you're creating a simple module. You'd also be able to accomplish what you want it via eval, but modules are cleaner.