From my AnnoCPAN comment:

These package blocks are only necessary if multiple classes are being defined in the same file, or there are mixed classes and code. (Which is what the synopsis shows.) They keep the lexical variables for each in separate scopes. If a single file holds only a single class, the enclosing file scope is sufficient and the braces aren't necessary.

Perl 6 uses a class declaration with braces. From Apocalypse 12, here's an example of a Perl 6 class:

# A Perl 6 class

class Point {
    has $.x;
    has $.y is rw;

    method clear () { $.x = 0; $.y = 0; }
}
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Since a Perl 5 package statement isn't exactly the same as a Perl 6 class statement, it's potentially confusing to write this:

# A Perl 5 class

package Point; {
    use Object::InsideOut;

    my @x :Field;
    my @y :Field('Accessor' => 'y');

    sub clear { 
        my $self = shift;
        $x[$$self] = 0; 
        $y[$$self] = 0; 
    }
}
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In Perl 5, the braces are just defining a lexical scope, which inside-out objects happen to use to create encapsulation. But unless there are other lexical scopes in the file that defines the Perl 5 class, the braces aren't necessary and writing them on the same line may make people think it's related to the package statement. As an example, consider this version with the braces moved:

# A Perl 5 class -- take 2

package Point; 

{
    # same guts as before
}

# still part of package Point outside the braces
# but @x and @y can't be seen from out here
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Now consider what happens if the package statement is inside the braces instead:

# A Perl 5 class -- take 3

package Foo; # just for later clarity

{
    package Point; 
    
    # same guts as before
}

# Back to package Foo out here
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