variable interpolation only occurs within doublequotes.

But careful, this example will not work, since you have to take care about escaping characters like backslashes!

UPDATE: IMHO the best solution is an explicit concat:

 open (MYFILE, '>>', 'Z:\My Documents\Workspace\' . "$test.csv" );

or

 open (MYFILE, '>>', 'Z:\My Documents\Workspace\' . $test . '.csv' );

(untested)