Some have already been mentioned, but:

• My template is:

  use 5.6.0;
  
  use strict;
  use warnings FATAL => 'all';
  
  __END__
  [download]

• Write everything outside-in, matching close to open immediately.
• Proper nesting (my editor helps)
• Syntax highlighting
• No code without a test
• No refactoring with many tests.

I have still never done the constant-on-the-left thing. I suppose I should ...