See the comments. A ref becomes an object after being blessed, as now Perl knows who/what it is. 
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();

[download]


Update

So your code is more simple than I thought, there is no OO involved.

But the underlying mistake looks the same to Perl. As your syntax made Perl thought it was a method call, but the host is an unblessed ref.

In reply to Re: Unblessed Reference Message by pg
in thread Unblessed Reference Message by b310

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