# creates a hash with all dates seen on @fileslist
my %seen_dates;
@seen_dates{ map { ( unpack( "x6A*", $_ ) )[0] } @fileslist } = ();

# increases the value of each seen @datelist element on %seen_dates. 
foreach (@datelist) {
    if ( exists $seen_dates{$_} ) {
        $seen_dates{$_}++;
    }
}