LIMIT clauses can be different with placeholders. The number is sometimes considered not a normal data value. Some databases, like MS-SQL, don't support placeholders in the LIMIT clause.
With PostgreSQL and DBD::Pg, the placeholders are substituted on the client. Without bind_param types, they use strings which are converted for all normal types. This doesn't work for LIMIT clause which must be an unquoted number. Supposedly, setting the type with bind_param will work:
my $sth = $dbh->prepare("SELECT * FROM big_table LIMIT ?");
$sth->bind_param(1, $value, SQL_INTEGER);
I don't know what will happen when the server-side parameters are implemented.