"On input, [*pcchBuf] is the full size of the buffer, including a space for a terminating null character.", so you're telling Windows $path has can accommodate 0 bytes. (A very useful trick is to call such functions twice, once to get the size needed, and once with the proper buffer size.)

my( $path, $pathLength ) = ( "\0" x 260, "\0" x 8 );
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should be

use constant MAX_PATH => 260;

my $path       = "\0" x MAX_PATH;
my $pathLength = pack('L', MAX_PATH);
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By the way, 8 bytes was longer than needed.