This sounds like a case for composition and delegation instead of inheritance.

Create a Net::FTP object in your constructor, but don't inherit from Net::FTP. Add an AUTOLOAD that dispatches all appropriate calls to the object. Something like this untested code would work:

sub AUTOLOAD
{
    our $AUTOLOAD;
    my $self        = shift;
    my ($meth_name) = $AUTOLOAD =~ /::(\w+)$/;
    my $method      = $self->{_ftp}->can( $meth_name );
    return $self->{_ftp}->$method( @_ ) if $method;

    # handle error here
}
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