perl -MO=[-q,]Backend[,OPTIONS] foo.pl
If you pass the "-q" option to the module, then the STDOUT filehandle will be redirected into the variable $O::BEGIN_output during compilation. This has the effect that any output printed to STDOUT by BEGIN blocks or use'd modules will be stored in this variable rather than printed. It's useful with those backends which produce output themselves ("Deparse", "Concise" etc), so that their output is not confused with that generated by the code being compiled.
The command-line mentioned in the SYNOPSIS section corresponds to the Perl code
use O ("Backend", OPTIONS);
The "O::import" function loads the appropriate "B::Backend" module and calls its "compile" function, 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 a CHECK 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.pl
that 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.