I don't use DBD::mysql these days but I do maintain DBD::ODBC and help maintain DBD::Oracle. I'm not aware of anything in DBI which "tampers" in any way with strings passed to the prepare or bind_param methods.
I've looked at DBD::mysql code and I cannot see anything that explains your "it's as if you actually passed". The original poster said his inserts were with the do method and no bound params and as far as I can see if he'd done that with DBD::mysql (instead of postgres) the string would have ended up in the mysql client API mysql_stmt_prepare untouched. Are you saying bound parameters work differently in DBD::mysql?
Other than some rather strange enbabling/disabling of SET NAMES in the code I can only find a few calls to sv_utf8_decode to decode UTF8 data received from mysql server and a test of:
in the quote method.
I'm really interested in this from the point of another DBD maintainer.
In reply to Re^2: DBD::Pg encodes Perlstring to UTF-8 bytes instead of WIN1252 regardless client encoding