Contrary to what jcwren says, I would hardly label myself a regex guru, but thanks for the compliment :)

As far as I can tell, you can't use the match operator to return the length. The closest I came up with is a variation of Ozymandias' response:

$string = "This is a test\nMore Test"; 
$len++ for ($string =~ /./sg);
print $len;
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In the Perl Cookbook, it has a recipe for finding the nth occurence of a match, but that also uses a loop. If you could enumerate matches with a straight regex, you could construct a well-defined regex and skip the loop in the Cookbook.

Believe me, I tried :) If any monks would like to tackle this, I'd love to be proven wrong (and I'm sure it would be something ridiculously simple).

Update: And in my quest to come up with horribly unoptimized code:

$_ = "This is a test\nMore Test";
$len = (split //);
print $len;
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Or you can use this beauty (no, I'm not serious):

$string = "This is a test\nMore Test";
$len = (grep /./s, (@chars = split //, $string));
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No, I don't suggest using them. I just had to toss it out because no one else had mentioned it :)