I would need to keep that counts somewhere for another calculation but I would like to keep only the last 8 counts

If the script is being executed each hour (presumably via a cron job) and you need to retain/use the array from the prior run, then you might want to look at using the Storable module to store/retrieve the array.

Update:
Just in case you weren't sure what I was suggesting, here's a short test script example.

use strict;
use warnings;
use Storable qw(store retrieve);

my @cnt = -e 'file_count' ? @{ retrieve('file_count') } : ('') x 8;

my $fcount;
$fcount++ for glob('*');

unshift @cnt}, $fcount;
pop @cnt;

store(\@cnt, 'file_count');
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Edit: changed the array reference $cnt_aref to a plain array @cnt