Parts of Perl you never knew existed

This is a list of some of the darker, scarier, or lesser known areas of Perl. I am going to be grouping them into a set of talks to scare, disgust, and delight the DC Perl Mongers. There is no ordering to them In this post; I am just trying to start a list of talk items.

• Prototypes
• what happens when you add a & to the start of a function call (aka why you shouldn't)
• split in void context
• goto function
• values vs variables (a superset of lists vs arrays)
• wantarray and contexts (void, scalar, list)
• pseudohashes
• tie
• typeglobs
• PadWalker
• source filters
• adding methods to existing classes
• interpolating things that aren't scalars or arrays
• when to use each vs keys
• warnings
  • how to disable certain warnings without affecting other warnings
  • how to add categories
• B::Deparse, showing the magic Perl does for you
  • while (<>), while (readline), and while(readdir)
  • default values for split, print, etc.
• do file
• weird operators
• special variables
  • %main::
  • $]
  • $"
  • $,
  • $/ and $\
  • %SIG
• what strict does and why
  • symbolic references
  • barewords
• regex magic
  • \k
  • \g
  • \G
  • \Q
  • differences between scalar and list context
  • recursion

Should PERL_UNICODE be considered harmful?

I set PERL_UNICODE to "SDL" as a matter of course when setting up my environment. This means that all of my filehandles will use the UTF-8 PerlIO layer unless the locale says otherwise or a specific layer is chosen explicitly. I do this because I don't want to have to worry about calling binmode or explicitly setting the PerlIO layer when opening a file:

open my $fh, "<:utf8", $filename
    or die "could not open $filename: $!"

This has worked fine for me for years; however, recently I have noticed a few problems with it:

• You cannot compile Perl will it set
• Many modules fail their tests when it is set
• Scripts that work just fine in your environment fail in other environments
• since it affects all filehandles, it could cause bugs in modules (I have never actually seen this)

Given these issues, I am starting to consider PERL_UNICODE harmful and thinking about giving it up.

Does anyone know of any arguments to keep using it, or, conversely, more arguments to stop using it?