Use canonical path for Perl

[github at samba.org]

New comment by sathieu on Samba Github repository

https://github.com/samba-team/samba/pull/209#issuecomment-423969795
Comment:
I didn't expect opposition on this patch. Let's give some more remarks:
- Current codebase has 66 uses of `#!/usr/bin/perl` and 6 of `env.*perl`
- Nobody complained about those 66
- Using perl from `PATH` can break things (if a module is not found for example)
- Even a perl dev wants Samba to work, even if the /usr/local perl install is broken
- You can always call a script with `/usr/local/bin/perl script-name`