Don't use Ruby's Perl-style global variables

…if you can avoid it, like $: or $<. Instead, use their english aliases like $LOAD_PATH or $DEFAULT_INPUT.

One caveat: with the exception of $LOAD_PATH (an alias for $:), $LOADED_FEATURES ($") and $PROGRAM_NAME ($0), you need to require "english" before you can use the english aliases. I'd love to see english included into Ruby itself.

The English RDocs contain a list with all the available aliases.

And while we're at it:

• Use $stdout/$stderr/$stdin instead of STDOUT/STDERR/STDIN. STDOUT/STDERR/STDIN are constants, and while you can actually reassign constants in Ruby, you shouldn't. If you rely on the global variables instead, it's much easier to redirect $stdout to something else, for example.
• Use warn instead of $stderr.puts. That's what warn is there for, after all. And it allows you to surpress warnings if you really need to (by setting the warn level to 0 via -W0).
• Use abort if you need to immediately terminate for some reason.
• Use Integer and Float if you need to make sure the string you're converting actually contains an integer or a float. Observe:
  >> "1.23".to_f
  1.23
  >> Float("1.23")
  1.23
  >> "not a float".to_f
  0.0
  >> Float("not a float")
  ArgumentError: invalid value for Float(): "not a float"
  >> "not an integer".to_i
  0
  >> Integer("not an integer")
  ArgumentError: invalid value for Integer(): "not an integer"

Refer to Christian Neukirchen's Ruby Style Guide for more general info on how you can (and probably should) format your Ruby code.

(Hat-tip to @manveru, who pointed out the incorrect usage of STDOUT/STDERR/STDIN)