You probably meant my $number=shift; (not $my number=shift; — though that would produce a different error message).

Other than that, I don't see anything wrong with testing not defined ($array[$number]) to do what you want, and in fact it does work just fine for me... (in other words, I suppose the problem lies elsewhere).

#!/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.
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