ap.pm:
package ap;

sub new {
    my $self = {};
    bless $self; #comment this will cause the error you described.
    return $self;
}

sub item_id {
    my $self = shift;
    if (@_) {
        $self->{ITEM_ID} = shift;
    } 
    return $self->{ITEM_ID};
}

1;

ap.pl:
use ap;
use strict;

my $a = new ap;
$a->item_id(10);
print $a->item_id();