my $re= qr{\[\[            # opening delimiter
(               # capture...
  (?:           # a group
    (?!\]\])    #    that does not start with ]]
    .           #    and is a single character long
  )+            # and many of these groups, at least one.
)
]]              # closing delimiter
}x;

while ($test =~ m{$re}g) {
  print "FOO:<$1>\n";
}