deleting from the memory

[programmer.perl]

Hello,

I'm having trouble with a deleting the element of an array, here I'm attaching the output (error in the last line, if you see there are two commas), it seems that element is not being deleted, instead it's undefined (func. undef). How to delete the element (from memory)? I tried to look into perldoc but I couldn't find another functions except 'delete' and 'undef'. Here in perldoc about 'delete' : WARNING: Calling delete on array values is deprecated and likely to be removed in a future version of Perl.

Output of program:

Here are the statistics for the Sino.
Address: kk ID: kk 
<...continues...>
Course(s): Chem, Bio, Maths, C++, Java 
After dropping: Chem, Bio, Maths, , Java

Code of program and module:

package Student3; 
sub new { my $class = shift; my $ref = {}; 
  # Anonymous hash 
bless($ref, $class); 
return $ref; } 
sub set_student { 
my $self = shift; 
<...continues...>
print "Enter the student's course(s) delimiting them 
  with a comma (,) "; 
chomp(my $cour = <STDIN>); 
$cour =~ s/\s+//g; 
my @cour = split(",", $cour); 
chomp($self->{"Course(s)"}->{"cour"}=[@cour]); 
<...continues here...> 
} 
<...continues...>
sub drop_courses { 
   my $self=shift; 
   my $del_cour = shift; 
                   # $student1->drop_courses(["C++"]); 
   my $num = @{$del_cour}; 
   if ($num == 1) { 
       foreach $k (@{$self->{'Course(s)'}->{'cour'}}) { 
         if ($k eq $del_cour->[0]) { 
                $k=""; 
         } 
        } 
} 
print "After dropping: ", 
     join (", ", @{$self->{'Course(s)'}->{'cour'}}), "\n"; 
1;

The code of a program

#!/usr/bin/perl -w 
use Student3; 
use warnings; 
use strict; 
my $student1 = Student3->new; 
$student1->set_student; 
$student1->show_student; 
$student1->add_courses(["C++", "Java"]);
$student1->show_student; 
$student1->drop_courses(["C++"]);