Perl doesn't support nested sub-routines natively, but you can fake them by using functions defined at runtime.

sub a {
    local *b = sub {
        return 123;
    };

    return b();  # Works as expected
}

b();  # Error: "Undefined subroutine &main::b called at ..."

Some times nested functions can be useful for simplifying readability of code while also allowing simple copy/paste for code re-use.