'x' + char

\x65

irb> [0x65].pack('U')
#=> "e"
irb> "e"[0]
#=> 101

irb> "\x#{65}"
#=> "\x65"
irb> "\x65".split('')
#=> ["\", "x", "6", "5"]

File.open("foo.tmp", "w") do |f|
  f.set_encoding(Encoding::BINARY) # set_encoding is Ruby 1.9
  f.binmode                        # only useful on Windows
  f.putc "e".hex
end

irb(main):002:0> '65'.hex.chr
=> "e"

irb(main):003:0> ['65'.hex].pack("C")
=> "e"
irb(main):004:0> ['66', '6f', '6f'].map {|x| x.hex}.pack("C*")
=> "foo"