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use Benchmark;
$str1 = "_test (folloed by 1 or more spaces)";
...
{
    $str2 =~ /_test(?:\s+)(?!<)/;
}

Benchmark: timing 1000000 iterations of p1, p2, p3, p4...
        p1:  3 wallclock secs ( 3.00 usr +  0.00 sys =  3.00 CPU)
...
             @ 323624.60/s (n=1000000)
        p4:  3 wallclock secs ( 2.82 usr +  0.00 sys =  2.82 CPU)
             @ 354609.93/s (n=1000000)