I do understand what's happening, but...

shadowing_outer_local_variable.rb

if false
  a = 2
  p a
else
  Proc.new do |a|
    p a
  end
end

not_shadowing_outer_local_variable.rb

if true
  Proc.new do |a|
    p a
  end
else
  a = 2
  p a
end

result

% ruby -w shadowing_outer_local_variable.rb
shadowing_outer_local_variable.rb:5: warning: shadowing outer local variable - a

% ruby -w not_shadowing_outer_local_variable.rb

(No warning!)