I don't know if this is documented, but I just changed it into a string reference (e.g. \" = drivers.userid" and it gave what I think you want: 
my($stmt2, @bind2) =
  $sql->select("users, drivers",
               ["users.username"],
               { "drivers.name" => "John",
                 "users.userid" => \"= drivers.userid"}
   );
print Dumper(\$stmt2, \@bind2);
# produces:
$VAR1 = \'SELECT users.username FROM users, drivers WHERE ( drivers.na
+me = ? AND users.userid = drivers.userid )';
$VAR2 = [
          'John'
        ];

[download]

Update: It is documented, but it should be given it's own section and title so that it stands out more. 

Finally, sometimes only literal SQL will do. If you want to include literal
SQL verbatim, you can specify it as a scalar reference, namely:

    my $inn = 'is Not Null';
    my %where = (
        priority => { '<', 2 },
        requestor => \$inn
    );

In reply to Re: joining tables with SQL::Abstract by pc88mxer
in thread joining tables with SQL::Abstract by citromatik

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