The problem here, is that you're feeding 'Delta_Days' an array that contains a comma separated string. Which is why it doesn't work - you're doing:

Delta_Days ( "2013,06,06", "2013,06,10" );
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You need another 'split' and do so on a 'comma'. E.g.:

my ( $firstdate, $seconddate ) = split ( /\t/ );
my @dates1 = split ( ",", $firstdate );
my @dates2 = split ( ",", $seconddate );
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You could probably do a bit more to smooth it out a bit further, but that's where you're going wrong. (I think - I've not run this code)