proc test1 {oper elem value}  {
    switch -exact $oper { add { lappend ::env($elem) $value } }
}
proc test2 {oper elem value}  {
    switch -exact -- $oper { add { lappend ::env($elem) $value } }
}
test1 add key1 "new value"
test2 add key2 "new value"