Run it through rxrx and see what happens, or even

$ perl -Mre=debug -e" $_ =qq{foo    bar \ntest};  m/(?<!foo)\s+bar\s+t
+est/g "
Compiling REx "(?<!foo)\s+bar\s+test"
Final program:
   1: UNLESSM[-3] (7)
   3:   EXACT <foo> (5)
   5:   SUCCEED (0)
   6: TAIL (7)
   7: PLUS (9)
   8:   SPACE (0)
   9: EXACT <bar> (11)
  11: PLUS (13)
  12:   SPACE (0)
  13: EXACT <test> (15)
  15: END (0)
floating "test" at 5..2147483647 (checking floating) minlen 9
Guessing start of match in sv for REx "(?<!foo)\s+bar\s+test" against 
+"foo    bar %ntest"
Found floating substr "test" at offset 12...
Guessed: match at offset 0
Matching REx "(?<!foo)\s+bar\s+test" against "foo    bar %ntest"
   0 <> <foo    bar>         |  1:UNLESSM[-3](7)
   0 <> <foo    bar>         |  7:PLUS(9)
                                  SPACE can match 0 times out of 21474
+83647...
                                  failed...
   1 <f> <oo    bar >        |  1:UNLESSM[-3](7)
   1 <f> <oo    bar >        |  7:PLUS(9)
                                  SPACE can match 0 times out of 21474
+83647...
                                  failed...
   2 <fo> <o    bar >        |  1:UNLESSM[-3](7)
   2 <fo> <o    bar >        |  7:PLUS(9)
                                  SPACE can match 0 times out of 21474
+83647...
                                  failed...
   3 <foo> <    bar %n>      |  1:UNLESSM[-3](7)
   0 <> <foo    bar>         |  3:  EXACT <foo>(5)
   3 <foo> <    bar %n>      |  5:  SUCCEED(0)
                                    subpattern success...
                                  failed...
   4 <foo > <   bar %nt>     |  1:UNLESSM[-3](7)
   1 <f> <oo    bar >        |  3:  EXACT <foo>(5)
                                    failed...
   4 <foo > <   bar %nt>     |  7:PLUS(9)
                                  SPACE can match 3 times out of 21474
+83647...
   7 <o    > <bar %ntest>    |  9:  EXACT <bar>(11)
  10 <   bar> < %ntest>      | 11:  PLUS(13)
                                    SPACE can match 2 times out of 214
+7483647...
  12 <   bar %n> <test>      | 13:    EXACT <test>(15)
  16 <   bar %ntest> <>      | 15:    END(0)
Match successful!
Freeing REx: "(?<!foo)\s+bar\s+test"
[download]

First it matches "foo" which isn't allowed, but then it matches "foo " which is allowed since its not "foo"

Basically, you need to rethink what you're trying to achieve, see Look Behind issues