The server means that you are not allowed to authorize on the plain connection but that you need to first upgrade the connection to TLS using the AUTH TLS command.

Starting with Version 3.0 Net::FTP has TLS included if you have a recent version of IO::Socket::SSL installed. In this case you need to just add a starttls call before you login:

use Net::FTP 3.01;
$ftp = Net::FTP->new(...) or die;
$ftp->starttls or die;
$ftp->login(...);
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