Update: Ignore this post. I am/was rushed, and I falsely thought it would help when I composed my real reply.

Quick question first: Are you amiable to changing

my $addr = mapFile( ... );
substr( $addr, 32, 4 ) =
    pack 'V', 1+ unpack( 'V', substr $addr, 32, 4 );
[download]

to

mapFile( my $addr, ... );
substr( $addr, 32, 4 ) =
    pack 'V', 1+ unpack( 'V', substr $addr, 32, 4 );
[download]

That's how tie gets around the problem.