perl -MO=Backend[,OPTIONS] foo.pl
The command-line mentioned in the SYNOPSIS section corresponds to the Perl code
use O ("Backend", OPTIONS);The import function which that calls loads in the appropriate B::Backend module and calls the compile function in that package, passing it OPTIONS. That function is expected to return a sub reference which we'll call CALLBACK. Next, the ``compile-only'' flag is switched on (equivalent to the command-line option -c) and an END block is registered which calls CALLBACK. Thus the main Perl program mentioned on the command-line is read in, parsed and compiled into internal syntax tree form. Since the -c flag is set, the program does not start running (excepting BEGIN blocks of course) but the CALLBACK function registered by the compiler backend is called.
In summary, a compiler backend module should be called ``B::Foo'' for some foo and live in the appropriate directory for that name. It should define a function called compile. When the user types
perl -MO=Foo,OPTIONS foo.plthat function is called and is passed those OPTIONS (split on commas). It should return a sub ref to the main compilation function. After the user's program is loaded and parsed, that returned sub ref is invoked which can then go ahead and do the compilation, usually by making use of the B module's functionality.