I guess it depends on what you mean by "External". The Benchmark module is part of the core Perl distro, and does exactly what you want. (from the pod):

    use Benchmark;
    $t0 = new Benchmark;
    # ... your code here ...
    $t1 = new Benchmark;
    $td = timediff($t1, $t0);
    print "the code took:",timestr($td),"\n";
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