ikegami is correct in that the syntax under question is documented as having undefined behavior.

That said, the way it generally behaves is:

• my has both compile-time and run-time behaviors. The compile-time behavior allocates the space for $string. The run-time behavior will assign the value on the RHS of the operator to $string.
• STATMENT if COND will perform the run-time portions of STATEMENT if COND evaluates to true ... at RUNTIME.

Generally, this trick is used to declare and initialize static variables within functions. Your second try is much better and cleaner, anyways.

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