I believe that the "SQL abstraction" provided by DBIx::Class is just SQL::Abstract. If that is true, then you can supply function calls as in the documentation:

\$sql_string
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So maybe the following works:

  my $rs = $self->db->resultset('MODELCLASS')->search(undef, {
    select => [
        'col1', 'col2', \"concat( date, ' ', time )",
      ],
      as => [ qw/
        id title time
      /],
      rows => 10
    });
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