use strict;
use warnings;

my $thr;

# ...

sub manager() {
    local $| = 1;
    
    while (print '.') {
        opendir(my $dir_handle, './acc'); 
        
        FILE:
        while (my $file = readdir($dir_handle) ) { 
            
            # or use /.dat/ if that's what you meant
            next FILE if $file !~ /[.]dat$/;
    
            open(my $reader, '<', "./acc/$file"); 
            my @items = <$reader>; 
            close($reader);
            
            $thr = threads->new(\&initiator, @items);
            unlink("./acc/$file");
        }
        
        close($dir_handle);        
        sleep(2);
    }
}