Why do you need to override the new method? Maybe this would work:

package Custom:FTP;
use base qw( Net::FTP );

sub login {
  my $self = shift;
  my @opts = @_;
  if (_not_prod()) {
    $opts[0] = 'testuser';
    $opts[1] = 'testpassword';
  }
  return $self->SUPER::login(@opts);
}
sub _not_prod {
  # check production
}
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I also agree with the poster above who mentioned using a config file. That seems the safer place to check if it's production, and just return different server/user/pass values if it is the devel server.