Blanks and underscores in html links

Look at
http://search.cpan.org/~rjbs/perl-5.12.3/pod/perlsyn.pod

There is a heading in the original source
=head2 Switch statements

The anchor that is generated somehow on the web is
<h2><a class='u' href='#___top' title='click to go to top of document'
name="Switch_statements_____"
 >Switch statements ...

Note that the space in the original is translated into an underscore, 
and the addition of several trailing underscores.  This means that the 
link on the page that goes like
See also L</"Switch statements">.

doesn't work, as it gets translated into
<a href="#Switch_statements" class="podlinkpod"

Can someone explain the trailing underscores?