DB<1> $_ = 'abcSUMcdexx'

  DB<2> s/(SUM.*)(..)/$1/

  DB<3> p
abcSUMcde
  DB<3> $_ = 'abcSUXcdexx'

  DB<4> s/(SUM.*)(..)/$1/

  DB<5> p
abcSUXcdexx
  DB<5>