Here is a solution also using Date::Calc.

#!/usr/bin/perl
use strict;
use warnings;
use 5.014;
use Date::Calc qw( Delta_Days Days_in_Month );

while (<DATA>) {
    my ($y1, $m1, $d1, $y2, $m2, $d2) = split /\D+/;
    $y2 ||= $y1;
    $m2 ||= $m1;
    $d2 ||= Days_in_Month($y2, $m2); # last day in current month
    say Delta_Days($y1, $m1, $d1, $y2, $m2, $d2);
}
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