In the statement 'foreach my $event (sort by_title $doc->findnodes( '//program' )) {do stuff}', the 'by_title' is the name of a subroutinE that exists elsewhere in your code. This subroutine is telling the sort command how to sort.

If you want to sort based upon other criteria using the same idiom, then you could make additional subroutines.

You can find out more on sorting here and by searching this site.