Making hacks file scoped like pragmas

The B::Hooks::OP::Check-based hacks are global, which means that they effect all code being used. For crazy hacks like PerlX::Range, it's better to make it localized like strict. Here's how I make it so (briefly.)

use B::Hooks::EndOfScope;

sub import {
    return if $^H{PerlXRange};

    $^H &= 0x00020000;
    $^H{PerlXRange} = 1;
  
    add_flop_hook();
    on_scope_end {
        remove_flop_hook();
    };
}

sub unimport {
    remove_flop_hook();
    $^H &= ~0x00020000;
    delete $^H{PerlXRange};
}

There are two special variables, $^H and %^H. They are really documented in the perlvar nicely, but not enough if you really want to play with it. Basically $^H is a bit vector, the bit 0x00020000 tells perl to localize the value of %^H for current package scope. In other words, the required/used code in other files will not see the value of $^H{PerlXRange}.

Then in the flop hook callback, checking if $^H{PerlXRange} exists first to tell if PerlX::Range is in the scope.

The unimport there is to let people saying no PerlX::Range in their code to turn it off explicitly.