DB<1> $foo = 'a=b';

  DB<2> $foo = 'x' . $foo

  DB<3> p scalar $foo =~/(?:a=([a-z]+))?/;
1
  DB<4> p $1

  DB<5>

  DB<6> p scalar $foo =~/(?:a=([a-z]+))?/g;
1
  DB<7> p pos $foo
0
  DB<8>