In the following mentioned code, I do understand what role undef has to play. 
my @found = $aggr_table->search(
         undef,
         {
             select => [
                 'span', 'max(ts)'
             ],
             as       => [qw/ span ts /],
             group_by => 'span',
         }

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Thanks in advance :) $aggr_table points to a table in the database  span and <code> ts</code are columns in that table

In reply to MySQL undef by akagrawal3

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