running B::Terse shows that the RHS is even precompiled and not evaluated at runtime:

lanx@lanx-1005HA:~$ perl -MO=Terse -e 's/(x)/$1.$1/re'
LISTOP (0x8eb6668) leave [1] 
    OP (0x8ec3040) enter 
    COP (0x8eb6688) nextstate 
    PMOP (0x8ebff20) subst 
        LOGOP (0x8ebff00) substcont 
            UNOP (0x8ebfbe8) null 
                LISTOP (0x8ebfc38) scope 
                    OP (0x8ebeb28) null [181] 
                    BINOP (0x8ebfc18) concat [3] 
                        UNOP (0x8ebfd30) null [15] 
                            PADOP (0x8ebfe20) gvsv  GV (0x8ef1c90) *1 
                        UNOP (0x8ebfc98) null [15] 
                            PADOP (0x8ebfcb8) gvsv  GV (0x8ef1c90) *1
[download]

That's why this rather ill construction works

lanx@lanx-1005HA:~$ perl -e 'sub bla { s/x/BEGIN{print "compiletime\n"
+}/e }'
compiletime
[download]

But still doesn't make the perldocs clearer...