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use strict;
use warnings;
...
# 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 );

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');

sub last_day_of_year {
   ( my $dt = DateTime->new( year => $_[0], month => 1, day => 1 ) )
...
for my $year (sort { $a <=> $b } keys(%in_range)) {
   say "$year: @{ $in_range{$year} }";
}