perl: use "use warnings" instead of -w

Change the Perl scripts to turn on lexical warnings instead of setting
the global $^W variable via the -w switch.

The -w sets warnings for all code that interpreter runs, while "use
warnings" is lexically scoped. The former is probably not what the
authors wanted.

As an auxiliary benefit it's now possible to build Git with:

    PERL_PATH='/usr/bin/env perl'

Which would previously result in failures, since "#!/usr/bin/env perl -w"
doesn't work as a shebang.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-svnimport.perl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 
 # This tool is copyright (c) 2005, Matthias Urlichs.
 # It is released under the Gnu Public License, version 2.

contrib/fast-import/import-directories.perl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 #
 # Copyright 2008-2009 Peter Krefting <peter@softwolves.pp.se>
 #
@@ -140,6 +140,7 @@ =head3 Revision commit message section
 
 # Globals
 use strict;
+use warnings;
 use integer;
 my $crlfmode = 0;
 my @revs;

git-add--interactive.perl

@@ -1,7 +1,8 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 
 use 5.008;
 use strict;
+use warnings;
 use Git;
 
 binmode(STDOUT, ":raw");

git-archimport.perl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 #
 # This tool is copyright (c) 2005, Martin Langhoff.
 # It is released under the Gnu Public License, version 2.

git-cvsexportcommit.perl

@@ -1,7 +1,8 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 
 use 5.008;
 use strict;
+use warnings;
 use Getopt::Std;
 use File::Temp qw(tempdir);
 use Data::Dumper;

git-cvsimport.perl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 
 # This tool is copyright (c) 2005, Matthias Urlichs.
 # It is released under the Gnu Public License, version 2.

git-send-email.perl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 #
 # Copyright 2002,2005 Greg Kroah-Hartman <greg@kroah.com>
 # Copyright 2005 Ryan Anderson <ryan@michonline.com>