# assuming @epoch_times contains the data

use constant ONE_DAY => 84_600; # seconds per day

my %per_day;
for my $time ( @epoch_times ) {
    my $key = int $time / ONE_DAY;
    if ( exists $per_day{ $key } ) {
        $per_day{ $key } = $time if $per_day{ $key } < $time;
        }
    else {
        $per_day{ $key } = $time;
        }
    }

@epoch_times = values %per_day;