This is an old question, but I saw that it was referenced on StackOverflow and had some bad comments.

You can get a list of columns by looking at 'NAME' and an array of primary flags by looking at 'mysql_is_pri_key'. Because multiple columns can be part of the primary, you need to look at each column in turn. This is the code I use in my generic table updater.

I am going to assume you already have a database handle ($dbh) and a table name ($table)

        my $sql = "select * from $table LIMIT 0,1";
        my $tnames = $dbh->prepare($sql);
           $tnames->execute();
        my $names = $tnames->{NAME};
        my $prime = $tnames->{'mysql_is_pri_key'};
        my $count = @{$names};
        foreach my $pos (0...($count-1)){
             print "Looking at column $names->[$pos] ";
             if($prime->[$pos] eq 1){
                   print "this column is part of the PRIMARY KEY\n";
                }else{
                   print "it is just a regular column\n";
                }
        }
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