I didn't say "perl can't do it."

I did say that I believe that you'll have better luck pursuing the answer from MicroSoft (since you have -- I trust -- read S:WE's documentation carefully and followed up with a look at on-line info such as http://homepage.eircom.net/~jmcnamara/perl/WriteExcel.html without finding any clues to implementing your desires).

I have come to that belief because Spreadsheet::WriteExcel was created to replicate (and AFAICT, is limited to) the internal file-writing capabilities of Excel itself. So investigating M$ for hints about how to use both line styles in one file might well tell you whether W:SE is likely to have that ability.

Or, of course, the module's author (jmcnamarra) may spot this thread and offer an authoritative answer (and that, of course, may differ substantially from mine).