As long as the insert and the select @@identity happen sequentially on the same connection you'll be fine:

$dbh->do("insert ... ");
$sth = $dbh->prepare('select @@identity');
$sth->execute();
my $data = $sth->fetch;...
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What will NOT work is selecting the @@identity value IF the insert happened in a stored procedure, because the @@identity variable is localized. So the following:

$dbh->do("exec some_proc_with_an_insert 1,2,3");
$sth = $dbh->prepare('select @@identity');
$sth->execute();
my $data = $sth->fetch;
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will NOT return the @@identity value generated in the stored procedure. This is the reason why you can't get the @@identity value after doing an insert with placeholders (at least with Sybase): any statement with placeholders generates a temporary stored procedure, hence localizing the @@identity variable.

Michael