Actually, when you look at it it isn't that surprising. Most likely the function overload::StrVal doesn't have a prototype. Therefore when you call it like that Perl parses it as a list operator (Update: The anonymous reply below is correct - the type of operator doesn't matter). That is to say, it is equivalent to

print overload::StrVal($froob . "");
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In other words, its taking the string value of $froob (which happens to be the return value of an overloaded stringification), appending a blank to it, and then calling StrVal, which does nothing since its argument is an ordinary string.