Until loop

an anyone tell me why the following gets stuck?

until ($ENV{'BlackHole$BinDir'} eq "") {
  system ('Unset BlackHole$BinDir');
}

It gets stuck in a loop as if the test is only tested once.

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Richard Torrens
4QD manufacture speed controllers for battery electric motors.
www sites http://www.4QD.co.uk http://www.4QDtec.com http://www.4QD.org
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