The lower-level loops are interrupted (that is, the loop is broken) when Perl detects that a repeated expression matched a zero-length substring. Thus:
m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;

   is made equivalent to

m{   (?: NON_ZERO_LENGTH )*
    |
     (?: ZERO_LENGTH )?
}x;

##</code><code>##

$_ = 'aaabbb';

/((?: a | c? )*)/x;       # 'c' matches once with zero-length
                          # which is not the same as
/(( a )* | ( c? )?)/x;    # where 'c' does not match at all