without using any additional module

Many of the useful functions come from the POSIX module, and you should have that already installed. Here is an example script which adds a number of days to current date:

#!/usr/bin/perl
use warnings;
use strict;
use POSIX qw(strftime);

my $calc_days;

if ( ! @ARGV ) {
   print "Please enter number of days (+/-): ";

   $calc_days = <STDIN>;
   chomp $calc_days;
   exit 1 if $calc_days eq '';
}
else {
   $calc_days = $ARGV[0];
}

# Convert days to seconds;
my $secs = $calc_days * 24 * 60 * 60;

# Convert seconds to a readable format, adding them to the current tim
+e
my $final_date = strftime ('%d/%m/%Y %T', localtime(time() + $secs));

print "$final_date\n";
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