package your::Net:FTP;
use strict;
use warnings;

# make your::Net::FTP inherit all the methods of Net::FTP
use base qw( Net:FTP );

# now you're free to redefine/override what ever you want

# our new method replacement
sub new {
  # call the original Net::FTP new method
  my $ftp_obj = shift->SUPER::new(@_) or return 0;

  # add our custom code
  $ftp_obj->{my_data} = 'test';

  return $ftp_obj;
}

# and method all our own
sub my_function {
  my $ftp_obj = shift;

  # do some stuff
}