C:\test>calc
Global symbol "$tree" requires explicit package name at C:\test\calc.pl line 8.
Global symbol "$tree" requires explicit package name at C:\test\calc.pl line 9.
....
Global symbol "@ops" requires explicit package name at C:\test\calc.pl line 77.
Global symbol "@ops" requires explicit package name at C:\test\calc.pl line 78.
Execution of C:\test\calc.pl aborted due to compilation errors.

C:\test>