use strict;
use warnings;
use feature qw( say );

use DateTime qw( );

# Or using your favourite DateTime::Format parser.
my $date1 = DateTime->new( year => 1981, month => 2, day => 1 );
my $date2 = DateTime->new( year => 1982, month => 2, day => 5 );

##</code><code>##

my $ref = DateTime->new( year => 1980, month => 1, day => 1 );
say $date1->delta_days($ref)->in_units('days');
say $date2->delta_days($ref)->in_units('days');

##</code><code>##

sub last_day_of_year {
   ( my $dt = DateTime->new( year => $_[0], month => 1, day => 1 ) )
      ->add( years => 1, days => -1 );
   return $dt->day_of_year();
}

my %in_range;
my $year1 = $date1->year();
my $year2 = $date2->year();
if ($year1 == $year2) {
   $in_range{$year1} = [ $date1->day_of_year()-1 .. $date2->day_of_year()-1 ];
} else {
   $in_range{$year1} = [ $date1->day_of_year()-1 .. last_day_of_year($year1)-1 ];
   for my $year ($year1+1 .. $year2-1) {
      $in_range{$year} = [ 0 .. last_day_of_year($year)-1 ];
   }
   $in_range{$year2} = [ 0 .. $date2->day_of_year()-1 ];
}

for my $year (sort { $a <=> $b } keys(%in_range)) {
   say "$year: @{ $in_range{$year} }";
}