Thank you Blue for posting what I did not have the time for.

I was in a similar situation for a long time in that I had code that needed to be run on various Unix boxes and NT in 1995. I found that find.pl did not work very well for me so I had to write my own that would work across Unix and NT; with or without Samba. (For some time I didn't even use Perl on NT, since it was too resource intensive and NT just couldn't deal with it.)
After that, I had the pleasure of needing code that worked on Linux, HP, BSD, Solaris, NCR, AIX, SCO, and Windows. By then the modules were good, but geting full coverage wasn't the easiest to ensure. I had already written a lot of code that were now covered by modules, but it meant going through and rewriting code to use modules. At the time, it was not worth it. I can estimate now that the test harness which had ~4,000 lines of code could now be written with ~1000 lines. I had started a rewrite late last year, but left the company before finishing.
What does mean for most people? Use modules if you can, but understand that not everyone has that luxury.