Portuguese, or Portugal citizens, are visa-exempt for Mozambique, but this does mean that they enter freely.
Nationals from visa-exempt countries must still:
Pay 650mtn (about $10USD) on arrival (take care of the receipt, you'll need it throughout your stay)
Show proof of accommodation reservations (paper copy)
Show they have means of subsistence (paper copy of bank extract)
Items 2 & 3 are not always required however it is best to have them to avoid potential hassles.
After paying 650mtn and stamping into Mozambique Portuguese citizens can stay up to 30 days and they can leave and return as many times as they wish during these 30 days.