I need more info about the context of the code you're going to write.

However, here's a possible solution:

my $os = eval "\\&FTP::Session::${^O}->new";
my %constor = ( $^O => $os );
[download]

I'm pretty sure that's what you're looking for.

Yes, the first argument is the package name, that's why constructors could look like this:

sub new { bless {}, shift };
# which is equivalent to this
sub new {
  my $class = shift @_;
  my $self  = {};
  my $ref   = bless $self, $class;
  return $ref;
}
# Bless could actually be:
bless {}, 'Package::Name';
[download]

Enjoy!

--
Casey