#!/bin/perl -l -pi.bak

use strict;
use warnings;

if (/Friday/) {
    warn "match on line $.\n";          # warn goes to STDERR
} elsif ($. == 2) {                     # no match but on line 2
    $_ .= "Friday";                     # add "Friday" (and line ending via -l)
    warn "added 'Friday' to line $.\n"; # warn doesn't add line ending via -l
}