if (...) $&Log ("it found base64-$i");

##</code><code>##

if($string_whole =~ m/
    ^ 
    (?: [A-Za-z0-9+/]{4} )*
    (?: [A-Za-z0-9+/]{2} [AEIMQUYcgkosw048] = 
    |   [A-Za-z0-9+/] [AQgw] ==
    )?
    \z
/x) {
   Log("$string_whole is valid base64");
}