Which tells me that the best thing would be to not try to guess at what version of Perl to draw the line at (or to at least guess low). Only draw a line if you at least know that nothing before that line works. For example, if you know that your module doesn't work before 5.005 (not a hard thing to know, since there are so many features that don't work before then, for example, for my ...), then feel free to say "BEGIN { require 5.005 } # No 'for my'" even though the module may not work in even 5.006.

Also feel free to not draw any line, IMHO. If Perl had a feature for declaring "I have tested this module as far back as version 5.006", then I'd encourage people to use it. I really need to write perlage.pod...