#use strict;
use warnings;
use Time::Local;
my @day_Of_Month={5,16,20};
#i want to use this array one by one to do unix time stamp

use Time::Piece;
my $t = localtime($^T);
print 'entire time:  ', $t;
print "\n";
print 'Month: ', $t->month;
print "\n";
print 'Year:  ', $t->year;
print "\n";
$updatetime = timegm(59,59,23,$dayOfMonth,$month-1,$year);
print $updatetime;
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im using this to create unix timestamp. but as im extracting the month and year, from localtime and the date of the month i have kept in the array that i will use to change to unix timestamp. how will i do unix timestamp for three given dates thats written in the array??

It looks like you want to create a series of timestamps, one for each day in the @day_of_Month array. After fixing the initialization as indicated by Bloodnok, you just need to loop through the days:

use strict;
use warnings;
use Time::Local;
my @day_Of_Month = ( 5, 16, 20 );
#i want to use this array one by one to do unix time stamp

use Time::Piece;
my $t = localtime($^T);
printf "entire time:  %s\n", $t;
printf "Month: %s\n",        $t->month;
printf "Year:  %s\n",        $t->year;

for my $dayOfMonth (@day_Of_Month) {
    my $updatetime = timegm(59,59,23,$dayOfMonth, $t->_mon, $t->year);
    printf "time for %2d  %d  (%s)\n", $dayOfMonth, $updatetime,
      scalar gmtime($updatetime);
}

#time for  5  1391644799  (Wed Feb  5 23:59:59 2014)
#time for 16  1392595199  (Sun Feb 16 23:59:59 2014)
#time for 20  1392940799  (Thu Feb 20 23:59:59 2014)
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If you really want to 'use this array one by one to do unix time stamp', it might pay you to change the initialization - for some reason, you're initialising the array with a hash ref (using a badly constructed hash definition, I might add) - I think you meant to write @day_Of_Month=(5,16,20); - quite what the purpose of this array is, I can't fathom from its' usage since the only reference is $day_Of_Month ... which doesn't exist.
A user level that continues to overstate my experience :-))

You don't really need any module other than core Time::Local.

use Time::Local;
use 5.010;

my ( $month, $year ) = ( gmtime($^T) )[ 4, 5 ];
my @month_days = ( 5, 16, 20 );

say timegm( 59, 59, 23, $_, $month, $year ) for @month_days;
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In addition, that sort of simplifies things, doesn't it?

Update: Linkified Time::Local

... stuff that is not "strftime" method/function

Hmm, my suggestion was to use strftime method, both Time::Piece and DateTime have one