use Data::Dumper;  $Data::Dumper::Useqq=1;

my $items = [ qw(foo bar baz) ];
my $num = "1 2\0";

@nums = split (' ', $num);
print Dumper @nums;
my $i = scalar @$items;
(warn "invalid items entered, try again\n")
    if grep (/\D/ || $_ < 1 || $_ > $i, @nums);

__END__
$ ./picklist-test.pl
$VAR1 = 1;
$VAR2 = "2\0";
invalid items entered, try again