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

.github/scripts/randcurl.pl

@@ -61,7 +61,7 @@ sub storedata {
 }
 
 sub getoptions {
-    my @all = `$curl --help all`;
+    my @all = qx($curl --help all);
     for my $o (@all) {
         chomp $o;
         if($o =~ /^ -(.), --([^ ]*) (.*)/) {