After reading the responses by ++haukex^{Re^4: this thread} and ++Discipulus^{Re^4: this thread}, I now have:

PERL5OPT=
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in .bash_profile (whence it's exported via set -a). And

alias perlo='perl -Mstrict -Mwarnings -Mautodie=:all'
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[perlo = perl + options] in .bash_aliases (which is sourced by .bash_profile).

While I don't think it would actually make any of my scripts "non-portable", I can foresee issues with 3rd party scripts: t/*.t scripts leapt to mind as an obvious example.

Thanks again for alerting me to this: I have discontinued using PERL5OPT.