Do you want to compare your dates in Perl or in SQL? You've received some good answers for comparing your dates in Perl. If you have a need to compare them in SQL, you would use Informix's string-to-date conversion and then compare the dates with the basic comparison operators.

I don't have experience with Informix; here's how it would look with DBI and Oracle, using TO_DATE to convert the string. This example looks for rows with a date_new column before 26/11/1987.

my $sth = $dbh->prepare(<<EndOfSQL);
  SELECT id
  FROM mytable
  WHERE date_new < TO_DATE(?, 'DD/MM/YYYY')
EndOfSQL

$sth->execute('26/11/1987');

while (my @row = $sth->fetchrow_array()) {
  print "@row\n";
}
[download]

I hope this is helpful; I would expect an Informix solution to look remarkably similar.