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

tests/sshhelp.pm

@@ -342,7 +342,7 @@ sub find_sshkeygen {
 sub find_httptlssrv {
     my $p = find_exe_file_hpath($httptlssrvexe);
     if($p) {
-        my @o = `"$p" -l`;
+        my @o = qx("$p" -l);
         my $found;
         for(@o) {
             if(/Key exchange: SRP/) {