An error I often make is forgetting semicolons. I do that most often after debugging print/printf/warn statements, or long statements, like ones that end with the closing brace of an anonymous sub. This, fortunately, causes a syntax error pointing to the right place in the majority of cases.

The other error I often do is failing to return the right value from a subroutine. The value is computed all right, only I forget to add the last statement that returns it after writing the body.