have you actually tried your code? because I got... 
Use of uninitialized value in concatenation (.) or string at - line 10
+.

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It works because the import warnings sentence inside the BEGIN block changes the value of ${^WARNING_BITS} at compile time.

In reply to Re^2: $^W or require warnings and import warnings; by salva
in thread $^W or require warnings and import warnings; by szabgab

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