use warnings;
use strict;
use 5.010;

my @some_array = (1, 2, undef, 4, 5);

for (my $ii = 0; $ii < scalar(@some_array); $ii++) {
    print "Value at position $ii: @{[$some_array[$ii] // '--undef--']}\n";
}

##</code><code>##

Value at position 0: 1
Value at position 1: 2
Value at position 2: --undef--
Value at position 3: 4
Value at position 4: 5