Just a nit-pick; string eval does not start a new perl interpreter, although it does need to compile the $string every time the eval statement is executed, which usually makes it slow compared to alternative strategies.

There are also some subtle issues and bugs with eval $string and scoping, but you're unlikely to run into them if you use eval $string sparingly. update: as nobull states: in this particular case, you've ran into one such issue: a lexical variable declared inside a string eval is not available outside of the eval statement :-)