The aliasing persists until the lvalue of the binding ceases to exist. Typically, I think, people will prefer to bind lexicals, so typically the scope will be till the end of their lexical block.

Would $bound := %data{$key} give me a different $bound as $key changed?

No. The binding is static and early. That is, the rvalue is evaluated, a reference is taken to the result, and that reference is installed in the lvalues symbol table entry.

Damian