Does sleep get interrupted when any signal is received in perl?

I have this simple perl daemon:

#!/usr/bin/perl

use strict;
use warnings;
use Proc::Daemon;

Proc::Daemon::Init;

my $continue = 1;

$SIG{TERM} = sub { $continue = 0 };
$SIG{USR1} = sub { do_process(1) };

# basic daemon                                                                                    

boxesd_log("started boxesd");

while ($continue) {
    do_process(0);
    sleep(30);
}

boxesd_log("finished boxesd");

exit(0);

# required subroutines                                                                            

sub do_process {
    my ($notified) = @_;
    boxesd_log("doing it $notified");
}

But there is something that is not working right.

When the daemon starts, it logs every 30 seconds without the notification as expected:

Sat Oct 30 21:05:47 2010 doing it 0
Sat Oct 30 21:06:17 2010 doing it 0
Sat Oct 30 21:06:47 2010 doing it 0

The problem comes when I send the USR1 signal to the process using kill -USR1 xxxxx. The output is not what I expect:

Sat Oct 30 21:08:25 2010 doing it 1
Sat Oct 30 21:08:25 2010 doing it 0

I get two continuous entries, one from the signal handling subroutine and another form the ever running loop. It seems as if the sleep gets interrupted whenever the USR1 signal is received.

What is going on?