doc: Try to find nonstandard Perl path from the environment

doc/texi2pod.pl

-#! /usr/bin/perl -w
+#!/usr/bin/env perl
 
 #   Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
 
@@ -23,6 +23,8 @@
 # markup to Perl POD format.  It's intended to be used to extract
 # something suitable for a manpage from a Texinfo document.
 
+use warnings;
+
 $output = 0;
 $skipping = 0;
 %sects = ();