my $names =
[
    {
        id   => 'X01',
        item => 'Widget',
    },
    {
        id   => 'Y02',
        xref =>  12,
        yref =>  13,
    },
    {
        item => 'none',
    },
    {
        id   =>  undef,
    },
]; 

##</code><code>##

foreach my $a (@{ $names }) {

   push(@ids, $a->{id} // ());
}

##</code><code>##

my $ids = join ',', map { $_->{id} // () } @$names;