my $n_secs = 30;  #seconds after which we die.
my $start = time;

## each iteration of this loop is short, but the whole
## thing is long
while ( $some_condition ) {
    die 'We ran too long!' if time-$start >= $n_secs;

    ## do stuff ##
}

##</code><code>##

package RunKiller;

sub new {
   my $self = shift;
   my $obj = { timer => time(), length => shift };

   bless $obj, $self;
   return $self;
}

sub check {
   my $self = shift;
   die 'Ran too long!' if time-($self->{timer}) >= $self->{length};
}

1;

##</code><code>##

require RunKiller;
my $runtimer = RunKiller->new(30); #30s run length.

while (1) {
   $runtimer->check();
   ## do something ##
}