from the DBI manpage:

The two most common uses of signals in relation to the DBI
are for canceling operations when the user types Ctrl-C
(interrupt), and for implementing a timeout using alarm()
and $SIG{ALRM}.

and according to the perlfunc manpage on alarm you would do something along these lines:

eval {
   local $SIG{ALRM} = sub {die "alarm\n";};
   alarm($timeout);
   $db->execute($query);
   alarm(0);
};
if ($@) {
   die "Error" unless $@ eq "alarm\n";
   #timed out
} else {
   #didnt
}
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