use Perl

Well yes, and they are called description files not Makefiles, but nomenclature aside, this is the one item on the list whose veracity I can vouch for -- the rest are just rumors :-).

That doesn't look like a proper importer because that's not the proper importer; this is:

http://mercurial.selenic.com/wiki/ConvertExtension

You basically just do

$ hg convert

in whatever repository you are converting from, but there are lots of options including some svn-specific features. I think it also remembers what it's converted so a subsequent convert picks up where it left off. I've never used it with Subversion, but I once converted the Perl git repository this way and it worked fine.