Just a bit more fodder ... I changed sub host_mask to print it's value if it was already set and as it returns it. And instead of referencing the package var directly in host_path -- am going through sub-host_mask:

  sub host_mask (;$) { my $p = shift;
    if ($pkg_host_mask) {
      P "(hostmask pre-set to %s)", $pkg_host_mask;
      return $pkg_host_mask;
    }
    $pkg_host_mask = $_[0] if @_;
    $pkg_host_mask
  }
[download]

Now the output seems to even more clearly show weirdness:

>Parseable::new, pckg= 'main{}'
 >URL::Fetchable::new
  >URL::Cacheable::new
   >URL::new, pkg='main'
   (SET) storage_path="cache"
   storage_path=cache
   <URL::new
(hostmask pre-set to (?-ims:([a-zA-Z][-a-zA-Z0-9]*\.[a-zA-Z]+)$))
(hostmask pre-set to (?-ims:([a-zA-Z][-a-zA-Z0-9]*\.[a-zA-Z]+)$))
   host_mask="(?-ims:([a-z...", 
   charset="UTF-8", 
(hostmask pre-set to (?-ims:([a-zA-Z][-a-zA-Z0-9]*\.[a-zA-Z]+)$))
(hostmask pre-set to (?-ims:([a-zA-Z][-a-zA-Z0-9]*\.[a-zA-Z]+)$))
$p->host_mask=(?-ims:([a-zA-Z][-a-zA-Z0-9]*\.[a-zA-Z]+)$), host=
(hostmask pre-set to (?-ims:([a-zA-Z][-a-zA-Z0-9]*\.[a-zA-Z]+)$))
Use of uninitialized value $host_mask in regexp compilation at ./crawl
+.pl line 400.
 at ./crawl.pl line 400.
        URL::Cacheable::host_path('main=HASH(0x168f338)') called at ./
+crawl.pl line 409
...
[download]

I mean how much more 'confidence' do I need to know host_mask really is set!!... yet still... the regexp compilation tosses it's cookies....!#$%~@#!*)&()+!
Oops, sorry, my fingers slipped must have slipped..