Building from Singularity recipes problems - perl

Question

I am a beginner in Singularity and I would like having help in building from a recipe.
I want to use GARM, a genome assemblies reconciliation tool, to reconciliate two genomes I have assembled. Since I am working on a HPC server, I don't have the permissions to install the required perl packages for this tool (which are Parallel::ForkManager and List::MoreUtils) and R. I found there are two possible solutions for that (and please tell me if I am wrong or if there are other things I can do):
install R and perl modules locally (but in this case I should change all the scripts references to them)
build a Singularity container containing all the dependencies needed and GARM, since the server has Singularity 2.6 installed.
I choosen the second solution and decided to use a recipe to build the container, which is the following:
Bootstrap: docker
From: ubuntu
%files
GARM_0.7.4
%post -c /bin/bash
apt-get update
apt-get install -y --force-yes perl build-essential cpanminus r-base
cpanm Parallel::ForkManager List::MoreUtils
export GARMBIN=/GARM_0.7.4/bin
export GARMLIB=/GARM_0.7.4/lib
export MUMBIN=/GARM_0.7.4/MUMmer3.22
export AMOSBIN=/GARM_0.7.4/amos-3.0.0/bin
export AMOSLIB=/GARM_0.7.4/amos-3.0.0/lib
%runscript
exec perl /GARM_0.7.4/GARM.pl
where GARM_0.7.4 is the folder downloaded containing everything the software needs, except R and perl modules.
I use sudo singularity build garm.simg recipe-garm to build the container.
Then using singularity run garm.simg --help to see if the container works, it comes out this:
Can't open perl script "/GARM_0.7.4/GARM.pl": Permission denied
Now, even if on the server I cannot use sudo, on my PC I tried anyway it with sudo singularity run garm.simg --help, and the output is:
Empty compile time value given to use lib at /GARM_0.7.4/GARM.pl line 12.
Can't locate GARM_main.pm in #INC (you may need to install the GARM_main module) (#INC contains: /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.30.0 /usr/local/share/perl/5.30.0 /usr/lib/x86_64-linux-gnu/perl5/5.30 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.30 /usr/share/perl/5.30 /usr/local/lib/site_perl /usr/lib/x86_64-linux-gnu/perl-base) at /GARM_0.7.4/GARM.pl line 13.
BEGIN failed--compilation aborted at /GARM_0.7.4/GARM.pl line 13.
I don't know what is wrong. I can't get if the problem is the garm folder not copied inside the container so that the garm script can't be run, or if there are sintax problem in the recipe and the container does not run the script.
EDIT: added exports as README.txt from the tool suggests, but the situation does not change.