I'm prepared to bet the error you got was: Can't use string ("3") as a SCALAR ref while "strict refs". This is one reason why your function isn't very friendly.

What your function is doing is _expecting_ a reference, then modifying the value referenced. So you can call it by:

foo ( \$data );
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Which is passing in a reference, that then "foo" alters. Because otherwise what you're doing is: ${'3'}++ which doesn't make a lot of sense.

Action at a distance like that is generally considered bad form - you don't know what what the subroutine "foo" is doing to your local variable. So normally, what you would expect to pass in a value, and get back a result.

You should probably note - you _do_ get a result back from your subroutine, that's the initial (pre-increment) value of $data. That can be confusing