A REPL environment is more of a tool chain issue than a language design issue.
Moreof, maybe, but it is language design, too. If you had a Java REPL, you still would have to type quite a bit of stuff (a class and a function) before you would have your "Hello, world".
Yes, you could add an implicit class named GLOBAL that magically gets a property named 'P' the moment you type
P = "Hello, world";
Most java code is valid Groovy code and it has some nice dynamic features, syntax sugar and has closures. I am currently using it to hack together a small DSL to easily configure a transformation from XML -> CSV
Here's an example of a Groovy session (From my instance of IntelliJ IDEA through Tools -> Groovy Console)
> P = "Hello world!"
Hello world!
> sub = P.substring(5)
world!
> sub ==~ / wo.+/ ? println("World") : {}()
WorldUp to Java 7 you could say "most" but why not "all"? Was it too difficult to test full compatibility? Why is the == slightly different between Java and Groovy, and the rest of the little differences which only confuse? And as for your example it doesn't look in the slightest like Java...
P = "Hello world!"
sub = P.substring(5)
sub ==~ / wo.+/ ? println("World") : {}()