It shows the representation of the code after it's been parsed.

From the B::Deparse documentation:

B::Deparse is a backend module for the Perl compiler that generates perl source code, based on the internal compiled structure that perl itself creates after parsing a program. The output of B::Deparse won't be exactly the same as the original source, since perl doesn't keep track of comments or whitespace, and there isn't a one-to-one correspondence between perl's syntactical constructions and their compiled form, but it will often be close. When you use the -p option, the output also includes parentheses even when they are not required by precedence, which can make it easy to see if perl is parsing your expressions the way you intended.

An explanation on why the O module is used, from a response to "B::Deparse vs. O=Deparse":

B::Deparse loads functions to deparse Perl code. But since you never use the module, it's rather useless to load the module.

O, on the other hand, behaves very specially when loaded. It loads the program, dumps it using the specified dumper (e.g B::Deparse), and prevents it from executing as if -c had been specified.