linecounter

#!perl

use strict; use warnings;

use Getopt::Std;
our ($opt_n, $opt_v);
getopts('vn:');

if ( $opt_v ) {
	warn "warning you every $opt_n lines";
}

my $counter;
while (<>) {
	$counter++;
	unless ( $counter % $opt_n ) {
		warn "Processed $counter lines";
	}
	print $_;
}

while (<>) {
    $counter++;
    warn "Processed $counter lines\n" unless $counter % $opt_n;
    print;
}

\n at the end of warn() text suppresses the "at line N ..." after each output
$_ at the end of print() can be omitted as print will implicitly use it.

or perl -ne 'BEGIN{$n=3} $c++; warn "Processed $c lines\n" unless $c % $n; print' filename.ext

-n -- assumes while(<>) code
-e -- oneliner
BEGIN {} block will be executed once before start