This is an example of using symbolic references (which is why you had to put the no strict 'refs' part in.)

And to keep things straight, remember that the $x=0 piece of code is referring to a package global, whereas the my $x=42; piece of code is creating a lexical scalar. The lexical is a different entity than the package global. Within the {...} block, the global $x is pretty much hidden by the lexical $x of the same name. And when everyone plays nice and avoids symbolic references, that's usually the behavior we see and expect; lexical scoping masking variables of the same name from broader-scoped blocks, and the broader-scoped lexicals (or globals) being protected from whatever the more narrowly scoped entities of the same name are doing.

But when you play with symbolic refs, you have to remember that the symbolic reference always refers to the package global, which in the example, equals 0, not 42.

This behavior is documented in perlref:

Only package variables (globals, even if localized) are visible to symbolic references. Lexical variables (declared with my()) aren't in a symbol table, and thus are invisible to this mechanism.

And the POD even provides a similar example:

local $value = 10;
    $ref = "value";
    {
    my $value = 20;
    print $$ref;
    }
__OUTPUT__
10
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