use Benchmark;
$str1 = "_test (folloed by 1 or more spaces)";
$str2 = "_test < xxx >";

timethese ( 1000000,
            { 'p1' => '&p1;',
              'p2' => '&p2;',
              'p3' => '&p3;',
              'p4' => '&p4;',
             } );

sub p1 ()
{
    $str1 =~ /_test(?>\s+)(?!<)/;
}

sub p2 ()
{
    $str1 =~ /_test(?:\s+)(?!<)/;
}

sub p3 ()
{
    $str2 =~ /_test(?>\s+)(?!<)/;
}

sub p4 ()
{
    $str2 =~ /_test(?:\s+)(?!<)/;
}