Efficient bit counting with a twist.

BrowserUk has asked for the wisdom of the Perl Monks concerning the following question:

Given a bit vector $v, this: my $c = unpack '%32b*', $v; will count the set bits very efficiently.

However, I need to count the set bits preceding (not including) a given bit position within $v.

I can do that using:

## $p = position
my $c = 0; vec( $v, $_, 1 ) and ++$c for 0 .. $p - 1;
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But that is grossly inefficient for large vectors.

Can you do it more efficiently?

Note:The rest of the code is pure perl, so I'd rather avoid going to XS to do this if possible.

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