ee Execute right side as code then eval the
resulting string.
##</code><code>##

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

##</code><code>##

lanx@lanx-1005HA:~$ perl -MO=Terse -e ' s/(x)/$1.$1/ree '
LISTOP (0x9f99668) leave [1] 
    OP (0x9fa6040) enter 
    COP (0x9f99688) nextstate 
    PMOP (0x9fa2f20) subst 
        LOGOP (0x9fa2e20) substcont 
            UNOP (0x9fa2f00) entereval [256] # --- string eval of result
                UNOP (0x9fa2c58) null              
                    LISTOP (0x9fa2be8) scope    # --- precompiled $1.$1
                        OP (0x9fa1b28) null [181] 
                        BINOP (0x9fa2c38) concat [3]  
                            UNOP (0x9fa2cb8) null [15] 
                                PADOP (0x9fa2d30) gvsv  GV (0x9fd4c90) *1 
                            UNOP (0x9fa2c18) null [15] 
                                PADOP (0x9fa2c98) gvsv  GV (0x9fd4c90) *1 
-e syntax OK