Hi

So it doesn't already show up in any of the generated makefiles?

One of CFLAGS/LDFLAGS would be the place to add it, possibly by editing postamble in https://metacpan.org/source/RSCHUPP/PAR-Packer-1.045/myldr/Makefile.PL

If its not specified in any makefile , https://stackoverflow.com/questions/25352389/what-is-the-difference-between-macosx-deployment-target-and-mmacosx-version-min/25362535 says you can specify the same thing by  export MACOSX_DEPLOYMENT_TARGET=10.6