Is MIME::Base64 installed?

MIME::Lite->send('smtp'... uses Net::SMTP, which in its auth method does:

  eval {
    require MIME::Base64;
    require Authen::SASL;
  } or $self->set_status(500, ["Need MIME::Base64 and Authen::SASL tod
+o auth"]), return 0;
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MIME::Lite code includes:

if ($smtp->supports('AUTH',500,["Command unknown: 'AUTH'"])) {
    $smtp->auth( $args{AuthUser}, $args{AuthPass} )
        or die "SMTP auth() command failed: $!\n"
            . $smtp->message . "\n";
} else {
    die "SMTP auth() command not supported on $hostname\n";
}
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It appears possible that the lack of MIME::Base64 could result in the error you're seeing.