Set perl path to /usr/bin/perl

ExtUtils::MakeMaker can change the hash-bang line to the perl that was
used when invoking Makefile.PL. It will however only change it if it is
something like #!perl or #!/usr/bin/perl; but not #!/usr/bin/env perl.

By changing the line to #!/usr/bin/perl then it will automatically be
rewritten to the correct perl when building.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>