This logic (corrected as per previous replies):

$col->[26] = (($col->[26] eq  '0') ? ''  :  $col->[26] );
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could also be expressed as:

$col->[26] ||= '';
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If the array element in question contains "0", that will evaluate to false, and the value will be set to the empty string; the assignment will also happen if the array element is undef (which might be a good thing to handle), and if it is already an empty string (which causes an insignificant redundancy).

But if I were expecting a field to contain an 8-digit date, I'd be tempted to be at least a little more explicit about that:

$col->[26] = '' unless ( $col->[26] =~ /^\d{8}$/ );
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That still allows dates like "00000000" (which would have been caught by the "||=" operator) or "99999999" (which would probably be easy to catch by using one of the Date::... modules to validate the value).

Deciding how careful you want to be is a question of trade-offs -- striking the right balance between flagrant carelessness and compulsive precision -- and how well you know your input data.