Technically, I think it depends on the specific DBD driver you're using what happens exactly when you're using placeholders, but one thing to consider that a statement using place-holders can be static and so only needs to be parsed once, which can mean considerable speedup.

For example:

my $sth = $dbh->prepare("SELECT something WHERE field=?");
for (@list_of_stuff) {
  $sth->execute($_);
  push @results,$sth->fetchrow_arrayref();
}
[download]

vs

for (@list_of_stuff) {
  my $sth = $dbh->prepare("SELECT something WHERE field=".$dbh->quote(
+$_));
  $sth->execute();
  push @results,$sth->fetchrow_arrayref();
}
[download]

Combine that with prepare_cached, and you can get probably see that there is a lot of potential for increased speed with placeholders. Especially if the database or its client library implements place holders natively (as I believe MySQL does).