#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
    chomp;
    print "with    (?>): '$_' matches\n" if /_test(?>\s+)(?!<)/;
    print "without (?>): '$_' matches\n" if /_test\s+(?!<)/;
}

__DATA__
_test foo
_test <
_test  <
_test<
[download]

Running this gives:

with    (?>): '_test foo' matches
without (?>): '_test foo' matches
without (?>): '_test  <' matches
[download]

Abigail