For me, it's simple. I usually use the documentation that's sitting right on my computer. But sometimes, I need more.

I only keep documentation for the version(s) of Perl that I'm running. It's occasionally quite useful to check the online docs for a different version, and compare feature sets.

If I find a bug in my version of Perl, switching versions may solve my problem. If a feature was changed in a given release, my code may need to change slightly as well. Good documentation can help in both cases.

Online documentation sites provide this information in an easy-to-browse format, for which I'm quite grateful. Downloading a full perl installation just to read documentation is a waste of time and bandwidth.
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Ytrew Q. Uiop