sub add_customer {
	# $data_hash is a hashref representing the fields 
	# in the customers table that will be filled.
	my ( $data_hash ) = @_;
	my ( $fields, $values, $place_holders ) = format_insert_data( $data_hash );
	my $sth = $dbh->prepare( "INSERT into customers ( $fields ) VALUES ( $place_holders )" );
	$sth->execute( @$values );
}

sub format_insert_data {
	# Takes a hash ref with keys being field names and
	# values being the values to insert into the database and
	# returns a list of fields, values, and placeholders suitable for an SQL INSERT statement:
	# INSERT into table ( $fields ) VALUES ( $place_holders )
	my ( $hash_ref ) = @_;
	if ( ref $hash_ref ne 'HASH' ) {
		croak "Argument to format_insert_data must be a hash reference";
	}
	my @fields = keys %$hash_ref;
	my $fields = join ',', @fields;
	my @values = @$hash_ref{ @fields };
	my $place_holders = '?,' x scalar @fields;
	chop $place_holders; # remove trailing comma
	return ( $fields, \@values, $place_holders );
}