mkdir -p fakemodules/Data
echo "package Data::Dumper;1;" > fakemodules/Data/Dumper.pm
# this will fail:
perl -Ifakemodules -MData::Dumper -e'print Data::Dumper::Dumper([1])'
a=$?
# this succeeds:
perl -MData::Dumper -e'print Data::Dumper::Dumper([1])'
b=$?
if [ $a -ne 0 ] && [ $b -eq 0 ]; then echo "module is required"; fi