Just inherit properly, using the glob as the underlying data structure. The only thing you'll have to pay attention to is how you access and set your data:

package My::FTP;
use base 'Net::FTP';

sub new {
# my($class,$host,%args) = @_;
  my $self = Net::FTP::new(@_) or die $@;
  ${*$self}{_custom_data} = 'CUSTOM DATA';
  return $self;
}

sub custom {
  my $self = shift;
  return ${*$self}{_custom_data},"\n";
}

package main;
my $ftp = My::FTP->new(shift);
$ftp->login(shift,shift) or die $ftp->message;
print $ftp->pwd,$/;
print $ftp->custom,$/;
print ref $ftp,$/;

__END__
/
CUSTOM DATA
My::FTP
[download]

Hope this helps.