The Geo::GDAL modules are the Perl bindings to the GDAL/OGR library. The modules allow you to access and manipulate from Perl all geospatial data that the installed GDAL library is configured to read/write.
Some arguments are optional and have a default value. This is illustrated like this:
SomeMethod(arg1, arg2 = 4);
arg1 is a required argument and an example or a variable with illustrative name is given
arg2 is optional and if left off, will get the value 4 (in this case)