You can compose a complex regex from simple ones. You don't need to use just one regex to do the all work, either, you can progress in steps:

#! /usr/bin/perl
use warnings;
use strict;

my $element = qr/(?: \w+ (?: \s+ is \s+ \w+ )? )/x;

while (<DATA>) {
  chomp;
  s/\s+/ /g;
  while (/\b test\s+ ( $element (?: , \s* $element )* )/xg) {
      my $match = $1;
      my @elements = split /\s*,\s*/, $match;
      shift @elements;
      s/\s+ is \s+ \w+//x for @elements;
      print "($_)" for @elements;
      print "\n";
  }
}

__DATA__
this line has nothing, nothing, nothing...
1 , 2, 3, 4 is four, 5, 6 test 00,11 is one,22,  33 is three,44,55 is 
+ the best, and  this  is not a test 111, 222, 333 as random  words to
+ finish
this should be a test, but nothing must be returned 4444, 7777, 9999 i
+s garbage
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