Why does ruby's '$:' or '$LOAD_PATH' change when I run it with sudo?

I am trying to run a ruby script as root. When I try to require a gem, ruby says it can't be found. This is because of $:

$ ruby -e "puts $:"
/opt/local/lib/ruby/gems/1.9.1/gems/activesupport-3.0.0.beta4/lib
/opt/local/lib/ruby/gems/1.9.1/gems/cgi_multipart_eof_fix-2.5.0/lib
/opt/local/lib/ruby/gems/1.9.1/gems/daemons-1.1.0/lib
/opt/local/lib/ruby/gems/1.9.1/gems/eventmachine-0.12.10/lib
/opt/local/lib/ruby/gems/1.9.1/gems/fastthread-1.0.7/lib
/opt/local/lib/ruby/gems/1.9.1/gems/fastthread-1.0.7/ext
/opt/local/lib/ruby/gems/1.9.1/gems/fastthread-1.0.7/bin
/opt/local/lib/ruby/gems/1.9.1/gems/gem_plugin-0.2.3/bin
/opt/local/lib/ruby/gems/1.9.1/gems/gem_plugin-0.2.3/lib
/opt/local/lib/ruby/gems/1.9.1/gems/json_pure-1.4.3/bin
/opt/local/lib/ruby/gems/1.9.1/gems/json_pure-1.4.3/lib
/opt/local/lib/ruby/gems/1.9.1/gems/mongrel-1.1.5/bin
/opt/local/lib/ruby/gems/1.9.1/gems/mongrel-1.1.5/lib
/opt/local/lib/ruby/gems/1.9.1/gems/msgpack-0.4.3/lib
/opt/local/lib/ruby/gems/1.9.1/gems/nestful-0.0.2/lib
/opt/local/lib/ruby/gems/1.9.1/gems/nokogiri-1.4.2/bin
/opt/local/lib/ruby/gems/1.9.1/gems/nokogiri-1.4.2/lib
/opt/local/lib/ruby/gems/1.9.1/gems/num_to_bytes-1.4.1/ext
/opt/local/lib/ruby/gems/1.9.1/gems/num_to_bytes-1.4.1/bin
/opt/local/lib/ruby/gems/1.9.1/gems/open4-1.0.1/lib
/opt/local/lib/ruby/gems/1.9.1/gems/rack-1.2.1/bin
/opt/local/lib/ruby/gems/1.9.1/gems/rack-1.2.1/lib
/opt/local/lib/ruby/gems/1.9.1/gems/rake-0.8.7/bin
/opt/local/lib/ruby/gems/1.9.1/gems/rake-0.8.7/lib
/opt/local/lib/ruby/gems/1.9.1/gems/ruby-processing-1.0.9/bin
/opt/local/lib/ruby/gems/1.9.1/gems/ruby-processing-1.0.9/lib
/opt/local/lib/ruby/gems/1.9.1/gems/ruby_parser-2.0.4/b开发者_如何学Cin
/opt/local/lib/ruby/gems/1.9.1/gems/ruby_parser-2.0.4/lib
/opt/local/lib/ruby/gems/1.9.1/gems/rubyforge-2.0.4/bin
/opt/local/lib/ruby/gems/1.9.1/gems/rubyforge-2.0.4/lib
/opt/local/lib/ruby/gems/1.9.1/gems/sexp_processor-3.0.4/lib
/opt/local/lib/ruby/gems/1.9.1/gems/sinatra-1.0/lib
/opt/local/lib/ruby/gems/1.9.1/gems/thin-1.2.7/bin
/opt/local/lib/ruby/gems/1.9.1/gems/thin-1.2.7/lib
/opt/local/lib/ruby1.9/site_ruby/1.9.1
/opt/local/lib/ruby1.9/site_ruby/1.9.1/i386-darwin10
/opt/local/lib/ruby1.9/site_ruby
/opt/local/lib/ruby1.9/vendor_ruby/1.9.1
/opt/local/lib/ruby1.9/vendor_ruby/1.9.1/i386-darwin10
/opt/local/lib/ruby1.9/vendor_ruby
/opt/local/lib/ruby1.9/1.9.1
/opt/local/lib/ruby1.9/1.9.1/i386-darwin10
.
$ sudo ruby -e "puts $:"
/opt/local/lib/ruby1.9/site_ruby/1.9.1
/opt/local/lib/ruby1.9/site_ruby/1.9.1/i386-darwin10
/opt/local/lib/ruby1.9/site_ruby
/opt/local/lib/ruby1.9/vendor_ruby/1.9.1
/opt/local/lib/ruby1.9/vendor_ruby/1.9.1/i386-darwin10
/opt/local/lib/ruby1.9/vendor_ruby
/opt/local/lib/ruby1.9/1.9.1
/opt/local/lib/ruby1.9/1.9.1/i386-darwin10
.

Why is this happening?

Try sudo -E ruby -e "puts $:".

From the sudo man page:

   -E          The -E (preserve environment) option will override the env_reset option in sudoers(5)).
               It is only available when either the matching command has the SETENV tag or the setenv
               option is set in sudoers(5).