#!/usr/bin/perl -w
print "File name to renumber:";
my $file = <STDIN>;
open (FILE, "$file") || die $!;
$|++;
my $line = 0;
while (<FILE>) {
	next if /^%/ ;
	s/^(?:N\d+\s?)?(?{$line++})/N$line / ;
}   continue {
	print FILE;
        close FILE;
}