DB<135> $_='x'
 => "x"

  DB<136> $a='$b'; $b="B"
 => "B"

  DB<137> s/x/$a.$a/r            # -> "$a.$a" (interpolation)
 => "\$b.\$b"

  DB<138> s/x/$a.$a/re           # -> eval '$a.$a'
 => "\$b\$b"

  DB<139> s/x/$a.$a/ree          # -> eval "\$b\$b" not eval "\$b.\$b"

Scalar found where operator expected at (eval 106)[(eval 105)[multi_perl5db.pl:644]:2] line 1, near "$b$b" ...