There are several ways to do it.

Since you want something standalone, without ties to the caller, a seperate process seems best. That allows the parent to use any timers and alarms it likes.

Lets call the thing "Timelimit", and abuse import to set the limit and fork, per Errto.

package Timelimit;
use warnings;
use strict;

=head1 NAME

Timelimit - fork off a watchdog timer

=cut

=head1 SYNOPSIS

  use Timelimit $seconds;

  use Timelimit;  # default 120 seconds

=cut

use vars qw/$N/;

sub import {
    my $class = shift;

    $N = 0+$_[0] || 120; # default two minutes

    my $ppid = $$;

    defined(my $pid = fork) or die $!;
    return $pid if $pid;

    local $SIG{HUP} = sub { exit 0 };

    while ($N > 0) {
        $N -= sleep $N;
    }

    kill INT, $ppid;
    # or, for maximum brutality,
    # kill KILL, $ppid;

    sleep 1 while kill 0, $ppid;
    exit 0;
}

1;

__END__

=head1 AUTHOR

Zaxo, Sept 2005

=cut
[download]

The initial $Timelimit::N setting is visible to the user who wants to see it.

Update ++Errto's suggestion gratefully adopted.