Can someone tell me why some CPAN modules have corresponding Linux (in my case, Ubuntu) packages?

They exist as Debian or Ubuntu packages only because someone packaged them. Perhaps they're very popular, or they're requirements of other packages. In general, there's no single reason.

In reply to Re^5: Install LWP from CPAN on Ubuntu Heron 8.04 by chromatic
in thread Install LWP from CPAN on Ubuntu Heron 8.04 by mpettis

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