you could just say:  my @deleted = $q->param('forum');.

But, you don't know if each delete was successful, so you'd have to keep track of that too. In fact, that would give you the chance to count (and report) both the successful and the failed deletes.

Something like this:

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 $@;
}

# ...
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