use Win32::OLE::Const;
# may need to adjust version number depending on what you have installed
my $const = Win32::OLE::Const->Load('Microsoft Office 12.0 Object Library');
...->AddShape($const->{msoShapeRectangle}, ...);

##</code><code>##

sub RGB { $_[0] | ($_[1] >> 8) | ($_[2] >> 16) }
my $shape = $slide->Shapes->AddLine(100,200,300,400);
$shape->{Line}{ForeColor}{RGB} = RGB(255,0,0);