If you mean SQL database when you say database, then you can ask the database directly with something like:

...
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";
[download]

Typically, database engines keep statistics about table columns so they can optimize queries, so the database engine many not even have to scan through the table to find all the values.

On the other hand, if you're using a flat file, or a data structure in memory, your approach is almost good. But rather than scanning all possible values, you could just scan the records and track the minimum and maximum values:

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";
[download]