>perl -MO=Concise -e"\substr($_,1)" 2>&1 | find "substr"
5           <@> substr[t4] sKRM/2 ->6
                              ^
                              |
                             "M"odifiable => magic created.

>perl -MO=Concise -e"$a = substr($_,1)" 2>&1 | find "substr"
5        <@> substr[t3] sK/2 ->6
                          ^
                          |
                          Not modifiable => no magic created.