I don't know if this is a terrible thing or an ignorable thing or somewhere in between

I would 'make install' it and see how it goes.
If the Pari stack keeps overflowing for you, then you'll need to take some action - either increase the allowable size with allocatemem (as suggested) or take some steps to find out more about what is going wrong.

For the latter, filing a bug report is probably a good start as the Math::Pari author is still quite responsive to such bug reports (I think).

Is 20 MB a reasonable maximum setting for the Pari stack ?

Cheers,
Rob