I'd like to use perl's format feature, but to have it also pad certain fields with underscores, eg to make header lines indicate the exact lenghts the values in the actual report lines take eg

format TOP = 
@|||||||||||||||||||  @|||||||||||||||||   @|||||||||
$c1, $c2, $c3
.

$c1 = "first column";
$c2 = " this month";
$c3 = " other data";
write;
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should come out as

____first column____  ____this month____   other data
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or (if spaces in the string could also be replaced:

____first_column____  ____this_month____   other_data
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The idea is that the underscores make it easier to see later how long a field Is this possible? Is there maybe an internal variable containing te character used for padding when '|' us used in a format description?