My first thought was also to do this in VB or VBA, but hey, this is the Perlmonks Monastery not the VB Nunnery (with my apologies to the members of the religious fairer sex).

If the existing templates are already set up to use the fields through some form of "mail merging" then all you have to do is have your Perl script spit out a "mail merge database file" and start MSWord through OLE and have it process the mail merge database file.

If mail merge is not an option, you're out-of-luck: you probably do not want to translate all existing template files into the MSWord HTML-format and then through some Templating system insert the right data at the appropriate places. As these templates are prone to be changed at any odd time, you will have to redo this exercise again and again.