I'd use Date::Calc:

    use Date::Calc 'Add_Delta_Days';
    my $yyyy_doy = "YYYYDOY";
    my ($year, $doy) = $yyyy_doy =~ /(....)(...)/;
    my ($YYYY, $MM, $DD) = Add_Delta_Days ($year, 1, 1, $doy - 1);
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What you want is DOY - 1 days after January 1 of the given year.

No doubt countless other modules on CPAN can do the same thing. But I like Date::Calc and that usually does what I need.