I am starting to suspect this code in s2/S2/BackendPerl.pm:

$io->writeln("package S2;");
$io->writeln("use strict;");
$io->writeln("use constant VTABLE => 0;");
$io->writeln("use constant STATIC => 1;");
$io->writeln("use constant PROPS => 2;");
$io->writeln("register_layer($this->{'layerID'});");

I think that results in the constants being redefined every time an S2 layer is loaded? Removing the lines had no immediate effect; I think all the layers would have to be recompiled as well?