Loop through the result set and add them to a data structure indexed on the contents of the first column.

my %items;

while (my ($item, $cost, $number, $value) = $sth->fetchrow_array())
{
    $items{ $item }{ cost }   += $cost;
    $items{ $item }{ number } += $number;
    $items{ $item }{ value }  += $value;
}
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Then just loop through the data structure and print appropriately.

You could also order the rows by the first column, keep a running tally in $cost, $number, and $value, and print a line when the first column changes.