I've ++ed this, but I wonder whether it isn't time to remove glob file handles from documentation and replace them with lexically bound file handles? Instead of:
open FILEHANDLE,'filename' or die "open of 'filename' failed: $!\n";
at least show the more modern idiom:
open my $handle,'<','filename' or die "open of 'filename' failed: $!\n
I realize the title is "Idioms explained", but you're also teaching people new idioms this way. So from a teaching point of view, I should at least mention that there is a new and better way of opening files since Perl 5.6.0 (if I'm not mistaken).