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