I simply read the OP as denoting uncertainty as to exactly what will happen if the same key is specified more-than-once in a hash initialization statement such as the one shown.   The definitive and now-documented answer seems to be:   that the initialization will reliably proceed left-to-right ... so that the rightmost occurrence of any key, by virtue of being the last assignment that is made for that particular key as the statement is executed, will always “win.”   Perl will not, so far as I know, generate any errors or warnings ... it probably isn’t what you meant to do, but it’s not illegal.