Having done this for a major internet company, let me tell you while the actual coding of exchange rates is rather straightforward, what exchange rate to use can be a complicated beast.

There is the interbank rate (nothing a joe monk would ever get), a typical credit card rate (normally interbank + 2%), a cash exchange rate (interbank + 4%), etc. Even then, some companies will not change their rates on a daily basis. They normally build in some type of buffer +-2% so their prices/costs don't look so volatile.

I'd recommend checking out the following sites: Oanda (which has a perl client) and Pacific Exchange Rate Service (which is related to the IMF in some manner).