O(1) memory solution (provided the results are grouped by stamp):

my $last;
my $count = 0;
while (my $row = $sth->fetch()) {
    my ($stamp, $status) = @_;

    if (defined($last) && $last ne $status) {
        print("$last: $count\n");
        $count = 0;
    }

    $last = $stamp;
    ++$count if $status eq 'In Use';
}

print("$last: $count\n") if defined($last);
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