Most likely you will need to use a module such as Win32::OLE, and reference the cell's comments using syntax such as (VBA):

Rng.Comment.Text
[download]

where Rng is a Range object referring to a single cell.

I think you might benefit from replies to the thread VBA to Perl; marto has some great links on translating VBA to Perl, and I've got a few references to some utilities as well.

An indispensable site for all things Excel- and spreadsheet-related is the aptly-named Spreadsheet Page, by John Walkenbach.

You may also wish to google for microsoft Excel MVPs, and extend your search from there.