If I understand what you want ( morning callers :), you can use the MySQL HOUR() function:

my $earlybirds =
    "SELECT
     foo, bar, baz
     FROM thedata
     WHERE HOUR(themoment) < 12";
[download]

You may want to replace '12' with ? if you will be interested in other times.

The MySQL docs, under 'Reference / Functions', will repay a browse. The BETWEEN() function would be useful for generalizing this query.

After Compline,
Zaxo