First you want to gather the information into a structure suitable for output. You want to output, for every product, quantity by rep. So, let's store it that way:

my %products;
for (@lines) {
  chomp;
  next unless my ($rep, $prod, $qty) = $_ =~ /(\w+)\s+(\w+)\s+(\d+)/;
  $product{$prod}{$rep} += $qty;
}
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Now you have a two-level hash of the data. At this point, I would suggest using Text::Table to produce the output. Failing that, it's really just: foreach product, foreach column, "print spaced out columns"