I'm not familiar with Oracle, but you should be able to use the compute clause in the select statement:

SELECT 
    report_no,
    reported_by,
    project_no,
    to_char(date_reported,'DD-MON-YYYY'),
    cat1, cat2, cat3
compute avg(cat1), avg(cat2), avg(cat3)
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If that doesn't work, try this:

before the first foreach loop:

my @sums;
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In the second loop:

$sums[$number] += $element;
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At the end you have the sum of each column in @sums and just have to divide each sum by $i, which holds the number of rows.