Not a real hidden secret, and mentioned on this site before: (however i use it, since it's save for my examle): the x operator.
have a look at the following example:

my $sql = join(",", @allsearchterms);
my $sth = $dbh->prepare("select * from table where value in($sql)");
# You need to place qutes arount the sql-items, but you'll get the poi
+nt here...
$sth->execute();
[download]

As you know, this isn't safe (sql Piggybacking)
Now concider the next code example:

my $qm = join ',', ('?') x @allsearchterms;
my $sth = $dbh->prepare("select * from table where value in($qm)");
$sth->execute(@allsearchterms);
[download]

As you see, a neat feature of Perl :)

Update:
code adjusted as suggested by Hue-Bond

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