Following up to my own question because I got an answer on #perl:

So, it is correct that the module's blib path would become blib/lib/SomeModule.pm, and in that case, the following is correct:

PM => {
    'lib/SomeModule.pm' => '$(INST_LIB)/SomeModule.pm',
},
[download]

If there's a separate POD file, the entries would be like this:

PM => {
    'lib/SomeModule.pm'  => '$(INST_LIB)/SomeModule.pm',
    'lib/SomeModule.pod' => '$(INST_LIB)/SomeModule.pod',
},
[download]

Thanks Grinnz on #perl, on irc.perl.org for the assistance.