grep /$string1.*$string2|$string2.*$string1/,

##</code><code>##

grep /
   ^
      (?=
         .* ($string1)
         (?{ 
            local $s1 = $-[1];
            local $e1 = $+[1];
         }) 
      )
      .* ($string2)
      (?(?{
         my $s2 = $-[2];
         my $e2 = $+[2];
         ($s2 < $e1) && ($e2 > $s1)
      })(?=A)(?!A))
/x,