This has nothing to do with placeholders. Try one of the following:

 $sth->execute( $id ) or err_trap;
 while ( my $row = $sth->fetchrow_arrayref ) {
    print "Email is: $row->[0]\n";
 }


 $sth->execute( $id ) or err_trap;
 while ( my ( $email ) = $sth->fetchrow_array ) {
    print "Email is: $email\n";
 }

 $sth->execute( $id ) or err_trap;
 my ( $email );
 $sth->bind_columns( \$email );
 while ( $sth->fetch ) {
    print "Email is: $email\n";
 }
[download]

Chris
M-x auto-bs-mode