Monkey patching `to_s` of Fixnum's class. Running it from the pry session as commands.

fixnum_pry_to_s.rb

puts `ruby -e 'class Fixnum; def to_s; "WHAT"; end; end; puts 42.to_s; puts "#{3 * 4}";'`
# WHAT
# 12

# this one works only in the pry
# .ruby -e 'class Fixnum; def to_s; "WHAT"; end; end; puts 42.to_s; puts "#{3 * 4}";'
# WHAT
# 12

system(%q{ruby -e 'class Fixnum; def to_s; "WHAT"; end; end; puts 42.to_s; puts "#{3 * 4}";'})
# WHAT
# WHAT

exec(%q{ruby -e 'class Fixnum; def to_s; "WHAT"; end; end; puts 42.to_s; puts "#{3 * 4}";'})
# WHAT
# WHAT