Since I cannot reopen #26. Setup lacking details.

[bib1963]

The ONLY files TIREX_BACKEND_MAP_CONFIGS is referenced in are:
/home/src/build/tirex/lib/Tirex/Backend.pm
/home/src/build/tirex/test/backend-test.sh
/home/src/build/tirex/backend-mapnik/renderd.cc
/home/src/build/tirex/backend-mapnik/test/test.sh

And the grep results:
/home/src/build/tirex/lib/Tirex/Backend.pm: my $mapfiles = $ENV{'TIREX_BACKEND_MAP_CONFIGS'} or $self->error_disable('missing TIREX_BACKEND_MAP_CONFIGS');
/home/src/build/tirex/test/backend-test.sh:export TIREX_BACKEND_MAP_CONFIGS="etc/renderer/test/checkerboard.conf.dist"
/home/src/build/tirex/bin/tirex-backend-manager: $ENV{'TIREX_BACKEND_MAP_CONFIGS'} = join(' ', map { $_->get_filename() } $renderer->get_maps());
/home/src/build/tirex/backend-mapnik/renderd.cc: tmp = getenv("TIREX_BACKEND_MAP_CONFIGS");
/home/src/build/tirex/backend-mapnik/test/test.sh:export TIREX_BACKEND_MAP_CONFIGS=test.conf

So, 2 of them are test files which set them up, 2 reference them, and one sets it up. NOWHERE is are these things described. You are not making things easy for people to get involved, since their first step is to get it up and running.

And the results of /opt/tirex/usr/bin/tirex-backend-manager -f --config /opt/tirex/etc/tirex/
Cannot load any Mapnik styles
Cannot load any Mapnik styles
Cannot load any Mapnik styles
Cannot load any Mapnik styles
Cannot load any Mapnik styles
Cannot load any Mapnik styles