DB<9> %options = ( r => sub { $_[0] =~ s/(a)/A/gr } ) # r is set

  DB<10> x @name = $options{r}->('abc')
0  'Abc'
  DB<11> $options{b} //= sub { $_[0] =~ s/(b)/B/gr }  # b defaulted if undefined 

  DB<12> x @name = $options{b}->('abc')
0  'aBc'
  DB<13> $options{r} //= sub { $_[0] =~ s/(b)/B/gr }  # r exists

  DB<14> x @name = $options{r}->('abc')
0  'Abc'
  DB<15>