...
use DBI;
...
my $DB = DBI->connect(...);
my $ar = $DB->selectall_arrayref(<<EOSQL);
   SELECT min(colName) AS min_val, max(colName) AS max_val
   FROM   tableName
EOSQL
print "Minimum value found: $$ar[0][0]\n"
.     "Maximum value found: $$ar[0][1]\n";

##</code><code>##

my ($min, $max);
for my $rec (@records) {
    $min = $$rec[$colNum] if !defined($min) or $$rec[$colNum]<$min;
    $max = $$rec[$colNum] if !defined($max) or $$rec[$colNum]>$max;
}
print "Values used are between $min and $max\n";