$ perl -MO=Terse -e '$var = "String"; $var = "foo$var"'
LISTOP leave [1]
    OP enter
    COP nextstate
    BINOP sassign
        SVOP const PV "String"
        SVOP gvsv  GV *var
    COP nextstate
    BINOP sassign
        BINOP concat [1]
            SVOP const PV "foo"
            SVOP gvsv  GV *var
        SVOP gvsv  GV *var

$ perl -MO=Terse -e '$var = "String"; $var = "foo" . $var' 
LISTOP leave [1]
    OP enter
    COP nextstate
    BINOP sassign
        SVOP const PV "String"
        SVOP gvsv  GV *var
    COP nextstate
    BINOP sassign
        BINOP concat [1]
            SVOP const PV "foo"
            SVOP gvsv  GV *var
        SVOP gvsv  GV *var