my @to_delete = $q->param('forum');
my ( @failed, @deleted );

local $dbh->{RaiseError} = 1;    # so we get exceptions for errors

foreach my $id (@to_delete) {
    eval {
        $dbh->do( q{ delete from mx_forums where id=? }, {}, $id );
        push @successful, $id;
    };

    push @failed, $id if $@;
}

# ...