perl: switch from backticks to qx()

To make it easier to find external command invocation in the source.

Also drop backticks from a comment.

To find external command invocations in Perl code use:
```sh
git grep -E "((exec|qx|open2|open3|system)\(|open\(.+-\|)" $(<perlfiles>)
```

Refs:
https://perldoc.perl.org/functions/qx
https://perldoc.perl.org/perlop#Simpler-Quote-Like-Operators

Closes #21994

scripts/randdisable

@@ -28,7 +28,7 @@ use List::Util qw/shuffle/;
 my @disable;
 
 sub getoptions {
-    my @all = `./configure --help`;
+    my @all = qx(./configure --help);
     for my $o (@all) {
         chomp $o;
         if($o =~ /(--disable-[^ ]*)/) {