#!/usr/bin/perl

use strict;
use warnings;

my @array;
$array[70] = "foo";

asubroutine(20);

sub asubroutine   {
    my $number=shift ;   # $number takes the value of 20
    if ( not defined ($array[$number]))    {    
       print "There is no such element $number in the array.\n";
    }
}

__END__
There is no such element 20 in the array.