Different kind of glob. Short for type glob, it's the name of the structure used for symbol table entries. Since each symbol name can be used to refer to a scalar, an array, a hash, a function, etc, a typeglob is a structure with a slot for scalar, a slot for an array, ... and a slot for an IO object. You are passing the glob, and the function grabs the associated IO object (or creates it in the case of open).
If, for a glob, Perl will check the IO::Handle namespace, then loading the IO::Handle module will populate that namespace with what Perl needs to find (in this case, the print method), yes?
Couldn't have said it better.