# make sure /tmp/unlocked_[ab] exist first

if ($write_to_A)
{
  lock_region {
    print "Writing to Drive A ...\n";
    ## do A stuff
    print "Done with Drive A.\n";
  } qw(/tmp/unlocked_a /tmp/locked_a);
}
else
{
  lock_region {
    print "Writing to Drive B ...\n";
    ## do B stuff
    print "Done with Drive B.\n";
  } qw(/tmp/unlocked_b /tmp/locked_b);
}