Following the constructive feedback here is the working code:

#!/bin/perl

use strict;
use warnings;


my $input_file = 'input.txt';
my $output_file = 'output.txt';


open(my $in_file,'<', $input_file) or die "Can not open file $input_fi
+le for writing: $!.\n";
open(my $out_file,'>', $output_file) or die "Can not open file $output
+_file for writing: $!.\n";

if (not (grep{/Monday/} <$in_file>) {

    seek $in_file, 0, 0;
    $. = 0;

    while (<$in_file>) {
        print $out_file $_;

        if ($. == 1) {
            print $out_file "Friday\n";
        }
    }
}

close($in_file);
close($out_file);
[download]

The same code but with 'unless' instead of 'if (not)'

#!/bin/perl

use strict;
use warnings;


my $input_file = 'input.txt';
my $output_file = 'output.txt';


open(my $in_file,'<', $input_file) or die "Can not open file $input_fi
+le for writing: $!.\n";
open(my $out_file,'>', $output_file) or die "Can not open file $output
+_file for writing: $!.\n";

unless (grep{/Monday/} <$in_file>) {

    seek $in_file, 0, 0;
    $. = 0;

    while (<$in_file>) {
        print $out_file $_;

        if ($. == 1) {
            print $out_file "Friday\n";
        }
    }
}

close($in_file);
close($out_file);
[download]

As 'seek' could probably be avoided if I rethink things in a different way, I'll get back to this when I have clarified my thoughts

I appriciated

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.