Are you saying that after this is executed:

    #And the code that should delete all the invalid input:
    my $i;
    foreach $i (@delete) {
        $query->delete($i);
    }
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That your $query object still have all of those params that you tried to delete? Test it and see.

    $query->delete_all(@delete);
    foreach my $deleted (@delete) {
        warn "Deleted ($deleted) NOT deleted" 
            if( $query->param($deleted );
    }
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If that executes fine with no warning about things not being deleted, then check where you are calling self_url().