Perl chooses "use more memory" in almost all design decisions. See (tye)Re: Generate all possibilities for a recent example of how much memory Perl can require to store a list.

It would probably be a lot of work to modify diagnostics.pm to run slower by not caching the modified contents of perldiag.pm in memory.

A few complaints were filed about recent suggestions to always use diagnostics or I would have filed one myself. The only value I see in diagnostics is after you've found an error that you don't understand or for abject newbies writing small code. I'd rather encourage the use of "-Mdiagnostics" when an error isn't understood.

Certainly you shouldn't be doing use diagnostics in production code.