bug#42953: Perl-mode: bonus spaces inserted

[積丹尼 Dan Jacobson]

In Perl mode, upon entering the ";" in

print "
";

Three spaces are wrongly inserted, making it

print "
    ";

I.e., type
 p [self-insert-command]
 r [self-insert-command]
 i [self-insert-command]
 n [self-insert-command]
 t [self-insert-command]
 SPC [self-insert-command]
 " [self-insert-command]
 RET [newline]
 " [self-insert-command]

OK, now type the

 ; [self-insert-command]

Whammo, three spaces are inserted.

emacs-version "26.3", using
$ emacs -Q -nw /tmp/q.pl

Sure "Just use \n. Case closed."
Yes, but emacs still has no business inserting spaces into strings.