#!/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 / ;
    print;
}
close (FILE);


##</code><code>##

 -i[extension]   edit <> files in place (makes backup
                 if extension supplied)
 -n              assume 'while (<>) { ... }' loop around program