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system("$useradd -c \"$fullname\" ...");
system( $useradd, qq|-c "fullname"|, ...
-c "value-of-$fullname"
system ($useradd, '-c', $fullname, ... '-p', $password, '-s', '/bin/false', '-u', $uid, $username);
system('cat -n /etc/group'); system('cat', '-n /etc/group'); system('cat', '-n', '/etc/group');