Here is the problem:
Fixnum = Fixnum<space>+<no space>Fixnum causes a
NameError
Fixnum = Fixnum<no space>+<no space>Fixnum  NO ERROR
Fixnum = Fixnum+Fixnum  NO ERROR
Same happens on '-','*'. I did not check the rest.

==============================================
irb(main):001:0> xx=0
0
irb(main):002:0> xx = xx + 1
1
irb(main):003:0> xx = xx +1
NameError: undefined method `xx' for
#<Object:0xa04a970>
        from (irb):3
irb(main):004:0>
irb(main):005:0> xx = xx+1
2
==================================================
irb(main):006:0> yy=9
9
irb(main):007:0> yy=yy - 1
8
irb(main):008:0> yy=yy -1
NameError: undefined method `yy' for
#<Object:0xa04a970>
        from (irb):8
irb(main):009:0> yy = yy - 1
7
==================================================

irb(main):010:0> zz = 1
1
irb(main):011:0> zz = zz * 2
2
irb(main):012:0> zz = zz *2
NameError: undefined method `zz' for
#<Object:0xa04a970>
        from (irb):12
irb(main):013:0> zz = zz*2
4
================================================


System info: Windows 2000 SP2

$ ruby -v
ruby 1.6.4 (2001-06-04) [i386-cygwin]

$ ls -l c:/ruby/bin/cygwin-ruby16.dll
-rwxr-xr-x    1 rlinder  None       564224 Jun  4 9:52
c:/ruby/bin/cygwin-ruby16.dll

$ md5sum
c:/ruby/bin/cygwin-ruby16.dllf3aa3c4a98f186fc808b9b3b3c49d8a8
*c:/ruby/bin/cygwin-ruby16.dll

from version.h
#define RUBY_VERSION "1.6.4"
#define RUBY_RELEASE_DATE "2001-06-04"
#define RUBY_VERSION_CODE 164
#define RUBY_RELEASE_CODE 20010604