Two things. First:

if ( $V_RoundedTotalNumberOfFreeGigabytes < 3.5 && 
          $V_RoundedTotalNumberOfFreeGigabytes == 3.5) {
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seems wrong. It can't be both < 3.5 and == 3.5 at the same time. Maybe you want to simply say <= 3.5 ?

Second, readdir returns just the filename. So on the unlink you probably want

unlink $dir_to_process . "/" . $file
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