e    Evaluate the right side as an expression.
ee   Evaluate the right side as a string then eval the result.

##</code><code>##

  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" ...

##</code><code>##

  DB<140> s/x/$a.'.'.$a/ree      # -> eval "\$b.\$b"
 => "BB"

  DB<141> s/x/$a.'.'.$a/re       # -> eval '$a.\'.\'.$a'
 => "\$b.\$b"

  DB<142> s/x/$a.'.'.$a/r        # -> "$a.'.'.$a"
 => "\$b.'.'.\$b"

  DB<143> eval "\$b.'.'.\$b"     # what perldoc predicts for line 140
 => "B.B"