Anonymous blocks can be nested in the procedure and exception blocks in as many levels as you want : Introduction « PL SQL Programming « Oracle PL/SQL Tutorial

1. Oracle PL/SQL Tutorial
2. PL SQL Programming
3. Introduction

<<MAIN>>
declare
    ...
     Declaration section
    ...
begin
    ...
    Procedural section
    ...
        <<SUB1>>
        declare
        ...
        begin
        ...
        end;
    ...
exception
    ...
end;

You can label all blocks including nested ones by using identifiers enclosed in << >>.

This notation allows programmers to reference elements of different blocks.