There are a number of ways you could do it:

$dup{$row[3]} = $dup{$row[3]} + 1; # Has trouble with undef
$dup{$row[3]} += 1;
++$dup{$row[3]};
$dup{$row[3]}++;
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+= and ++ are magical to the extent that they can update an undef value without complaint. That is often very useful, especially where a hash or array element is autovivified. For example, if you are using a hash to count the number of occurrences of words in a string you could:

++$wordCount{$_} for split ' ', $string;
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which generates an entry in %wordHash for each word and increments the count - very clear and concise.