use Time::Local;

my $date = '00250';
my ($yr, $day) = unpack('A2A3', $yr);

# Ugly hack to get round the fact that you have 
# two digit dates. Adjust cutoff to taste.
$yr += 100 if $yr < 70;

# Get epoch time at start of year.
my $when = timelocal(0, 0, 0, 1, 0, $yr);

# Increment by number of days
$when += 86_400 * $day;

# Convert back to MMDD
my ($d, $m) = (localtime($when))[3, 4];
++$m;

sprintf('%02d%02d', $d, $m);