I would just make:

 
sub Install_or_Upgrade{....}
[download]

and then:

sub install
{
   Install_or_Upgrade();
}
sub upgrade
{
   Install_or_Upgrade();
}
[download]

If this "Installer" or "Upgrader" is in a module, you just export "install" and "upgrade" and nobody has to know that they do the same thing.

Update: After awhile, if this is a significant application, the "Upgrader" will become a lot more complex than the "Installer".