my $sql = { SELECT => [ 'column1', 'column2', 'column3' ],
            FROM   => [ 'table1', 'table2' ],
            WHERE  => [ 'condition1', 'condition2' ],
            GROUP_BY => [ 'column1', 'column2' ],
            HAVING => [ 'condition3]' ],
            ORDER_BY => [ 'column1','column2' ],
            # ... you get the idea };

# Store the separators in another hash:
my $sep = { SELECT => ',',
            FROM   => ',',
            WHERE  => ' AND ',
            HAVING => ' AND ',
            GROUP_BY => ',',
            ORDER_BY => ',' };

##</code><code>##

my $query = join ' ', map { join $sep->{$_}, @{$sql->{$_}} } qw(SELECT FROM WHERE GROUP_BY HAVING);