#!/usr/bin/perl
use warnings;
use strict;

sub add_one_day {
    my $timestamp_file = 'timestamp.txt';

    open my $read_fh, '<', $timestamp_file;
    chomp ( my $timestamp = <$read_fh> );
    close $read_fh;

    $timestamp += 86400;

    open my $write_fh, '>', $timestamp_file;
    print   $write_fh "$timestamp\n";
    close   $write_fh;
}