Well, as long as you are shelling out with a system call, there is probably a native MacOS program to tell you the OS version ... I just always think of Perl solutions :-)

Update: sw_vers -productVersion seems to be the needed utility:

$ perl -E'say qx/sw_vers -productVersion/'
10.12.6
[download]

Note that this is a different value than that shown by using Config!

$ perl -MConfig -E'say $Config{osvers}'
16.7.0
[download]

The latter appears to be the macOS build version, in fact, and can be different from the time Perl was built to the present:

$ perl -E'say qx/sw_vers -buildVersion/'
16G1510
[download]

Hope this helps!