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/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 @@ by whitespace or other characters.
 
 # Globals
 use strict;
+use warnings;
 use integer;
 my $crlfmode = 0;
 my @revs;