There is a space after the (.)> in s/^(\d+\. )(.)_/ (where I put the underscore).

As for the right place for the margin settings, I would have supposed it should go into the prolog, but my experiments seem to indicate this particular setting is stored outside of the RTF file (perhaps in a stylesheet) (at least by WordPad) Aargh! reading the docs is better than experimenting (sometimes). A simple $rtf->print( \'\margl567\margr567' ); after the Prolog sets the left and right margin to 1cm.