On the latest commit (9a6a19d), installing ack modifies its install file in the _source-cache.

Steps to reproduce:

In a clean git repository:

$ spack fetch -D ack
[...]

$ head -n 1 var/spack/cache/_source-cache/archive/fd/fd0617585b88517a3d41d3d206c1dc38058c57b90dfd88c278049a41aeb5be38
#!/usr/bin/env perl

$ spack install ack
[...]

$ head -n 1 var/spack/cache/_source-cache/archive/fd/fd0617585b88517a3d41d3d206c1dc38058c57b90dfd88c278049a41aeb5be38
#!/path/to/local/repo/of/spack/opt/spack/linux-archrolling-skylake/gcc-9.2.0/perl-5.30.0-glhysleiayklpn7q76zpgc3grihti4j6/bin/perl

I do not know if there are other packages where this is an issue. We use a static download cache for fresh spack installations and ack was the first package to suddenly fail to install (after moving from this commit to that). I verified that the issue persists in develop until now.

I suspect that is related to the reorganization of the download cache in 4af4487.