A bare block is perfectly clear. It creates a limiting scope.

The BLOCK: block almost implies a loop, which it is, but that is not the intent. On the other hand, a more descriptive label like SCOPING_BLOCK: may work.

The do block by itself seems to be begging for while modifier. Assigning to a variable helps:

my $BigCalc = do {
    my $SmallCalc;
    ...
    f($SmallCalc);
};
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The if block looks like debugging code. Someone commented out a chunk of code with if (0) {...} then turned it back on.