Looks fine to me, though I don't know why you got DateTime involved to do what localtime does.

my @times = ...;

my %date_count;
foreach my $time (@times) {
   my ($y,$m,$d,$h) = (localtime($time))[5,4,3,2];
   $date_count{$y+1900}{$m+1}{$d}{$h}++;
}

...
[download]

I suppose you could use a flatter structure, but it's doesn't leave the data in a format as useful as yours.

use Time::Local qw( timelocal );

my @times = ...;

my %date_count;
foreach my $time (@times) {
   my ($y,$m,$d,$h) = (localtime($time))[5,4,3,2];
   $date_count{timelocal(0,0,0,$d,$m,$y)}++;
}

...
[download]