my $keys_ref = $dbh->selectall_arrayref("SHOW KEYS FROM $table");                  
    
foreach my $row ( @{$keys_ref} ) {
    print "Primary key: $row->[4]"
        if $row->[2] eq 'PRIMARY';
}                                 

##</code><code>##

$row->{'Column_name'} if $row->{'Key_name'} eq 'PRIMARY'