v:fname mods

I have a fullpath filename in v:fname minus the filetype, for example: A::B::C::Mymodule
 
How can I modify v:fname to contain: A/B/C/Mymodule/Mymodule.pm  (note: the last part of the path is duplicated before adding '.pm')
I used 'substitute' to replace '::' with '/'. I looked for a vim function to parse pathname in order to get the last part, but found nothing.
 
My ultimate goal is to set 'includeexpr' to an expression that will convert the object of a Perl "use" command to a filename for 'gf'.