I suspect you mean

                if (/^I/) {
                        read_cal(substr($_,1), $mode);
                } else {
                        print "|$_|\n";
                }
[download]

rather than

                if (/^I/) {
                        read_cal(substr($_,1), $mode);
                }
                print "|$_|\n";
[download]

This is because your $_ variable has been altered by the recursion, so using it is no longer appropriate! If you really want to keep it, you'll have to use a named, local variable.