my %constructor= {
        MSWin32 => sub { FTP::Session::Win32->new( @_ ) },
        HPUX => sub { FTP::Session::HPUX->new( @_ ) },
    }
    my $ctor= $constructor{$^O};
    die "No constructor for $^O"   if  ! $ctor;
    my $session= $ctor->( ... );
    # Old Perl requires:  $session= &$ctor( ... );