This sort of thing is trivial with a thread, a queue and some shared variables:

#! perl -slw
use strict;
use 5.010;
use threads;
use threads::shared;
use Thread::Queue;

sub tprint {
    my $tid = threads->tid;
    state $sem :shared;
    lock $sem;
    print "$tid: ",@_;
}

our $PERIOD //= 60;

my %db :shared;
my $Q = new Thread::Queue;

async {
    while( my( $expiry, $id ) = split ':', $Q->dequeue ) {
        my $now = time;
        sleep ( $expiry - $now ) if $now < $expiry;
        if( $db{ $id } == $expiry ) {
            lock %db;
            delete $db{ $id };
            tprint "User:$id expired:$expiry; deleted";
        }
        else {
            tprint "User:$id recently active; reprieved";
        }
    }
}->detach;

while( sleep 1 ) {
    my $id = int rand 100;
    my $expiry = time() + $PERIOD;
    if( exists $db{ $id } ) {
        tprint "Existing user:$id active; expiry extended until $expir
+y";
    }
    else {
        tprint "New user:$id added; expiry $expiry";
    }
    lock %db;
    $db{ $id } = $expiry;
    $Q->enqueue( "$expiry:$id" );
}

__END__
C:\test>889439 -PERIOD=10
0: New user:81 added; expiry 1298316449
0: New user:30 added; expiry 1298316450
0: New user:6 added; expiry 1298316451
0: New user:0 added; expiry 1298316452
0: New user:23 added; expiry 1298316453
0: New user:72 added; expiry 1298316454
0: New user:22 added; expiry 1298316455
0: New user:61 added; expiry 1298316456
0: New user:91 added; expiry 1298316457
0: New user:75 added; expiry 1298316458
1: User:81 expired:1298316449; deleted
0: New user:15 added; expiry 1298316459
1: User:30 expired:1298316450; deleted
0: New user:24 added; expiry 1298316460
1: User:6 expired:1298316451; deleted
0: New user:49 added; expiry 1298316461
1: User:0 expired:1298316452; deleted
0: Existing user:49 active; expiry extended until 1298316462
1: User:23 expired:1298316453; deleted
0: New user:87 added; expiry 1298316463
1: User:72 expired:1298316454; deleted
0: New user:19 added; expiry 1298316464
1: User:22 expired:1298316455; deleted
0: New user:40 added; expiry 1298316465
1: User:61 expired:1298316456; deleted
0: New user:13 added; expiry 1298316466
1: User:91 expired:1298316457; deleted
0: New user:78 added; expiry 1298316467
1: User:75 expired:1298316458; deleted
0: New user:98 added; expiry 1298316468
1: User:15 expired:1298316459; deleted
0: New user:53 added; expiry 1298316469
1: User:24 expired:1298316460; deleted
0: New user:20 added; expiry 1298316470
1: User:49 recently active; reprieved
0: New user:44 added; expiry 1298316471
1: User:49 expired:1298316462; deleted
0: New user:99 added; expiry 1298316472
1: User:87 expired:1298316463; deleted
0: New user:97 added; expiry 1298316473
1: User:19 expired:1298316464; deleted
0: Existing user:20 active; expiry extended until 1298316474
1: User:40 expired:1298316465; deleted
0: New user:24 added; expiry 1298316475
1: User:13 expired:1298316466; deleted
0: New user:70 added; expiry 1298316476
1: User:78 expired:1298316467; deleted
0: New user:87 added; expiry 1298316477
1: User:98 expired:1298316468; deleted
0: New user:42 added; expiry 1298316478
1: User:53 expired:1298316469; deleted
0: New user:79 added; expiry 1298316479
1: User:20 recently active; reprieved
0: New user:56 added; expiry 1298316480
1: User:44 expired:1298316471; deleted
0: New user:48 added; expiry 1298316481
1: User:99 expired:1298316472; deleted
0: Existing user:79 active; expiry extended until 1298316482
1: User:97 expired:1298316473; deleted
Terminating on signal SIGINT(2)
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