Another solution, with only one regex:

m/\d[a-zA-Z]|[a-zA-Z]\d/;
[download]

This works because in a string with both letters and numbers there has to be at least one location where a letter and a number touch

Update: To Laurent_R: Absolutely. Clarity and simplicity always wins. Except when this line is in the 3% of code that needs 99,7% of the runtime of a program and you have to optimise for speed