my @columns = (qw(col1 col2 col3));
my $query = "SELECT " . join (",", @columns) . " FROM tablename"; 
my $sth = $dbh->prepare($query);
$sth->execute;
while (my $href = $sth->fetchrow_hashref) {
    for (@columns) {
        print "$_ => $href->{$_}\n";
    }
}