print "<DIV style="display: none;">
my $count = 0;
while ( long_process_loop) { 
   # process
   ...
   ...
   $count++;
   if ( !($count % 1000) ) {
       print ".";
   }
}
print "</DIV>\n";
[download]

    my $OLD_SIG = $SIG{ALRM} if $SIG{ALRM};
    $SIG{ALRM} = sub { die "timeout" };

    eval {
        alarm( 30 );
        # do what needs to be done
        alarm( 0 );
    };

    if( $@ ) {
        if( $@ =~ /^timeout$/ ) {
            print STDERR "A timeout occurred";
            alarm(0);
        } else {
            print STDERR "Some other error occurred";
        }
    }

    $SIG{ALRM} = $OLD_SIG if $OLD_SIG;
[download]

-derby

update: Whoa! Mis-read your question as not more than 30 seconds. I'm not sure why you would want it to run for at least 30 seconds but you could sleep.

#!/usr/bin/perl -w
use strict;

my $time_to_die=0;
my $timeout=5; #in seconds 

$SIG{ALRM} = sub { $time_to_die=1; };

alarm($timeout);
while(!$time_to_die){
#tasks here 
print time,"\n";
}

#will allow while loop to complete 
#after $SIG{ALRM} is received
[download]

#!/usr/bin/perl -w
use strict;
my $start_time = time;
# All your other code goes here
my $end_time = time;
my $elapsed_time = $end_time - $start_time;
sleep 30 - $elapsed_time unless ($elapsed_time >= 30);
[download]

I am not exactly sure why you need your script to take at least this long to run - most people want to take less time. If this has something to do with monitoring the process or something - there is surely a better way to do it.

Hope this helps - cheers L~R

UPDATE: I just realized that you might want it to be allowed to run longer than 30 seconds. In that case, it is like perrin said - not a Perl issue. Someone/something else is imposing this arbitrary limit.