The idea is to protect dh-make-perl from DEP-5 changes
and to also make 'dh-make-perl refresh' respect more existing
content of a debian/copyright file. Specifically the new steps would be:

1.) Attempt to read the existing debian/copyright file into a
Debian::Copyright object.
2.) If 1. failed gather the data as one would at the moment and
create a Debian::Copyright object from that.
3.) Update the 'debian/*' stanza in the Debian::Copyright object.
4.) Refresh the common licenses in the Debian::Copyright object
5.) Rename any fields that have been changed according to the current
DEP-5 standard.
6.) Write out the new debian/copyright file.

Other comments:
a.) The main affect of the proposal would be that the manual bits of
a debian/copyright file would not get overwritten. Are there any downsides
to this?
b.) Debian::Copyright was modelled explicitly on Debian::Control. It would
need more work for all this to be doable however.