I think many of these short variable names were chosen to deal with the limits of interpreters on 1960s-ish machines. At that time, some of the principles underlying Huffman encoding were in play.

Perl just adopted many of these names as part of its mission. That Larry Wall chose to preserve the distinctive nature of this namespace and keep its expansion out of the spaces that would be commonly chosen for user tasks seems to me to be one of the insightful decisions in Perl's design. That it continues with ${^Word} style names is good.

Be well,
rir

Update: very minor language cleanup

Update: reworked poor sentence: the struck out stuff. You can't expand an ad hoc namespace because you're creating a new one. The underlined text replaces: add most of his other predefined variables into a namespace that ... words are failing me a bit, but he kept the namespace distinctive and