To debug the issue, I would print out @nums and @$items  (i.e. modify CPAN/FirstTime.pm¹).  Then you'll know which direction to look further.

Maybe there's some non-whitespace junk char in $num ... e.g.:

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
[download]

Or @$items has not enough elements (or is empty)...  There isn't really a lot that could go wrong in the test.

___

¹ If you don't know where it is,  perl -MCPAN::FirstTime -le 'print $INC{q(CPAN/FirstTime.pm)}' will tell you (use double quotes on Windows).