The code is actually sort of a failed attempt.

The author seems to realize that the use directive occurs at compiletime, and is not easily conditionalized. So he uses the runtime-executed 'require' method of invoking a module conditionally. However, he does it within a lexical block (  if(....){ lexical block } ). The warnings pragma is lexically scoped to the block in which it is invoked, so he is only turning warnings on for a brief instant, which will not have any effect outside the if conditional block. Even if it did extend past the if(){...} block, it would be limited by the lexical scope of the BEGIN{...} block.

You can observe this fact by modifying the code as follows:

BEGIN{
    if( $] >= 5.006_000 ) {
        require warnings;
        import warnings;
        print "Warnings on: $hi\n";
    } else {
        $^W = 1;
    }
}
print "Warnings off: $hi\n";
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With this code, the output will be something to the effect of:

Warnings on: 
Use of uninitialized value in mytest.pl, line xxx....
Warnings off:
[download]

In other words, the second use of uninitialized value is in a lexical scope not covered by warnings, and thus the use of uninitialized value is not reported the second time.