What's the purpose of perl's #line directives?

Question

Line directives (#line) are used to reconfigure perl's idea of the current filename and line number. When is this required to get right filename and line number (in an error message)?

Answer 1

Usually such markers are put into code that has been pre-processed or mechanically generated in order to refer back to the human-generated source.

For example, if there was a program that converted Python to Perl, it might insert a

# line 812 "foo.py"

so that error messages would refer to the original Python code which would make more sense to the programmer.

Answer 2

They're useful when wrapping a Perl script in another file, like pl2bat does. Perl doesn't see the batch commands at the beginning of the file which throws off its idea of line numbers. A #line directive at the beginning of the Perl source compensates for this.