gistfile1.txt

rbx-head :001 > foo
NameError: undefined local variable or method `foo' on an instance of Object.
   from Kernel(Object)#foo (method_missing) at kernel/delta/kernel.rb:85
   from { } in Object#irb_binding at (irb):1
rbx-head :002 > foo = 123 if false
 => nil 
rbx-head :003 > foo
 => nil 

Perl does the same thing. my $foo = 1 if false defines $foo, but its value is undef.