Execution proceeds as follows:

_('JUNK','rm','-rf','/')
  shift == 'JUNK' (true)
  &_ == _(@_) == _('rm','-rf','/')
    shift == 'rm' (true)
    &_ == _('-rf','/')
      shift == '-rf' (true)
      &_ == _('/')
        shift == '/' (true)
        &_ == _()
          shift == undef (false)
        &_ returns undef (false)
      &_ returns undef (false)
    &_ returns undef (false)
  &_ returns undef (false)
_ returns undef (false)
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so nothing after &_ is ever executed since _ never returns a true value.