Bash didn't exist when Perl was invented in 1987. Bash was first released two years later in 1989. In 1987 there was the Bourne shell sh, and there was csh and derivatives like tcsh, and there was the Korn shell ksh and its derivatives, and they each had their own unique quirky menageries of subtly nuanced punctuation and syntax and variable names. And of course there was sed, and awk, and C, and even BASIC.
Most if not all of the variables in question come from the Bourne shell. It's fair to mention bash still using them today because it's supposed to be Bourne's successor.
Plus, you can always "use English;" if you want less cryptic names.