The way to get a file from one filesystem to another is to copy it first and then unlink the original. If you "use File::Copy", there's a move function available to do it for you.

use File::Copy;

move '/fsa/file', '/fsb/file'
    or die "Can't move: $!\n";
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From a quick glance at File::Copy, it looks as if it will use rename, if possible.