I think your update to the OP came sometime after my first reply. I think your concerns are mostly answered by nothingmuch above, but to elaborate a bit, instead of this as your "more efficient" method:

chdir($workdir);
foreach ($files) {
   workOnFile();  ## should have $_ as an arg, shouldn't it?
}
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(where you might want each thread to have its own "$workdir"), you could do it like this:

foreach ( $files ) {
    workOnFile( "$workdir/$_" );
}
[download]

If you think that's less efficient, try benchmarking it and see how much of a hit you take. I haven't actually tested it, but I expect the hit won't amount to much, if it's noticeable at all.