I just want to mention Class::Trait. It might be interesting
Example: my executable:

#! /usr/bin/perl

use warnings ;
use MyModel ;
use newModel ;

my $m = MyModel->new() ;
Class::Trait->apply( $m, newModel ) ; # polymorphic stuff here :)

$m->test() ;
$m->newTest() ;
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and the the modules MyModule.pm

package MyModel ;

sub test {
  print "MyModel\n" ;
}

sub new {
  return bless {}, __PACKAGE__ ;
}
return 1 ;
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and newModel.pm:

package newModel ;

use Class::Trait 'base' ;

sub test {
  print "add test\n" ;
}

sub newTest {
  print "add new test\n" ;
}

return 1 ;
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Maybe not a very good example, but in your case you can replace MyModule (I think) with Net::FTP, this way you should be able to add new or replace existing subs in Net::FTP

Cheers
LuCa