How would you go about this?

I would avoid fork and threads in the same program--but then on my platform I avoid fork at almost all costs.

On my platform (windows), each thread has it's own current working directory:

c:\test>perl -Mthreads 
-le"@t = map{
    async(
        sub{ chdir shift;sleep 3;print`cd`}
    ,$_)
} qw[c:\\ c:\\test c:\\test\\_Inline];$_->join for @t"

c:\

c:\test

c:\test\_Inline
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But I seem to remember that doesn't hold true for other platforms.

In any case, I'd simply do:

system "cd $path && theProg @args";
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