I'll explain what I figured out.

First, what does do [...] do? Does it use something like "ARRAY(0x814bc28)" as the filename to open? If you look at the test file, at the top is:

unshift @INC, sub {
    no warnings 'uninitialized';
    ref $_[1] eq 'ARRAY' ? @{$_[1]} : $_[1];
};
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If you look in perldoc perlvar for the docs on @INC, you'll find that it says

You can also insert hooks into the file inclusion system by putting Perl code directly into @INC. Those hooks may be subroutine references, array references or blessed objects. See "require" in perlfunc for details.

So look also in perldoc perlfunc, grep for require VERSION followed by hooks. By this point my head is already spinning, but then note that before the do comes a 3-arg open of a reference to a heredoc:

open $fh, "<", \<<'EOC';
BEGIN {prepend_block_counting_filter};
pas("SSS make s fast SSS");
EOC
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The docs for that are in perldoc -f open:

open file handles to "in memory" files held in Perl scalars via:

    open($fh, ’>’, \$variable)
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Then note that \"foo" makes a reference to a string, so \<<'EOC';... is just a funny reference to a string that happens to've been made by a heredoc.

Now's where I get lost. Anyone fill in the details? (What's the $fh doing in do [$fh, sub {...}], for example?)