#!/bin/perl

use strict;
use warnings;


my $input_file = 'input.txt';


open(my $in_file,'<',$input_file) or die "Can not open file $input_file for reading: $!.\n";

if (grep{/Friday/} $in_file) { # WRONG. You are grepping the filehandle,
                               # not the content of the file.
    print "Match\n";
} else {
    print "No match\n";
}

print "Before\n";             # "Before" is long ago.

while (<$in_file>) {
    print;                    # you don't look for "Friday" looping lines.
}

print "After\n";              # after "After" nothing happens but file closing.

close($in_file);

##</code><code>##

#!/bin/perl

use strict;
use warnings;

my $input_file = 'input.txt';

open(my $in_file,'<',$input_file) or die "Can not open file $input_file for reading: $!.\n";

while (<$in_file>) {
    if (/Friday/) {
        warn "match on line $.\n"; # warn goes to STDERR
    } elsif ($. == 2) {            # no match on line 2
        chomp;                     # remove line ending
        $_ .= "Friday\n";          # add "Friday" and line ending
        warn "added 'Friday' to line $.\n";
    }
    print;
}

close($in_file);

##</code><code>##

#!/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
}