my @ids       = qw(a b c d e f g h);
my $chosen_id = '';

# Flat file!
# each line is a time and an id separated by a space
open my $recent_fh, '<', 'recent'
    or die "Can't read recent: $!";
my @recent_lines = <$recent_fh>;
close $recent_fh or die "Can't close recent: $!";
chomp @recent_lines;
my %recent_ids = map { split } @recent_lines;

# remove the recent ids from the id list
my @possible_ids = grep { ! $recent_ids{$_} } @ids;
$chosen_id = $possible_ids[rand @possible_ids];

# process it
# remove the oldest, as before

open $recent_fh, '>', 'recent'
    or die "Can't write recent: $!";
print $recent_fh
    map { "$_ " . $recent_ids{$_} . "\n" }
        keys %recent_ids;
close $recent_fh or die "Can't close recent: $!";