Use canonical path for Perl

[github at samba.org]

New comment by bjacke on Samba Github repository

https://github.com/samba-team/samba/pull/209#issuecomment-423953488
Comment:
just that most script use /usr/bin/perl instead of "/usr/bin/env perl" doesn't mean this this is the better thing to do. IMHO we should use "/usr/bin/env perl"  everywhere to support systems where perl is not installed in /usr/bin/