use strict;
use warnings;

my %day_of_week_order = ( Monday =>1, Tuesday => 2, Wednesday =>3,
                  Thursday => 4, Friday => 5, Saturday => 6,
                  Sunday => 7);

my %days = map{chomp; $_=>1}<DATA>;

$days{Friday} = 1; # make sure Friday is there

foreach my $day (sort{my $A = $day_of_week_order{$a};
                      my $B = $day_of_week_order{$b};
                            $A <=> $B } keys %days)
{
   print "$day\n";
}  

=prints
Monday
Friday
Saturday
Sunday
=cut
                 
__DATA__
Monday
Saturday
Sunday

##</code><code>##

exists ($days{Friday}) ? print "Friday was there\n" 
                       : print "Friday wasn't there\n";