Perl DBD and an alarming problem

I am mystified by the behavior of "alarm" in conjunction with Postgres/perl/DBD.  Here is roughly what I'm doing:

   eval {
      local $SIG{ALRM} = sub {die("Timeout");};
      $time = gettimeofday;
      alarm 20;
      $sth = $dbh->prepare("a query that may take a long time...");
      $sth->execute();
      alarm 0;
   };
   if ($@ && $@ =~ /Timeout/) {
      my $elapsed = gettimeofday - $time;
      print "Timed out after $elapsed seconds";
   }

Now the mystery: It works, but it hardly matters what time I use for the alarm call, the actual alarm event always
happensat 26 seconds.  I can set "alarm 1" or "alarm 20", and it almost always hits right at 26 seconds. 

Now if I increase alarm to anything in the range of about 25-60 seconds, the actual alarm arrives somewhere around the
90second mark.  It seems as though there are "windows of opportunity" for the alarm, and it is ignored until those
"windows"arrive. 

Anyone have a clue what's going on and/or how I can fix it?

A secondary question: It appears that $sth->cancel() is not implemented in the Pg DBD module.  Is that true?

Thanks,
Craig