use Date::Calc qw(Add_Delta_Days);

my $input       = "11004";                            # yyddd
$input          =~ /(\d\d)(\d\d\d)/;
my ($y, $m, $d) = Add_Delta_Days("20$1",1,1, $2 -1);  # based on an example in the Date::Calc-docu
my $output      = sprintf("%4d%02d%02d", $y, $m, $d);