#!/usr/bin/perl

use strict;
use POSIX;

my $sigset = POSIX::SigSet->new(&POSIX::SIGUSR1);
my $action = POSIX::SigAction->new(\&trap, $sigset, &POSIX::SA_SIGINFO);
POSIX::sigaction(&POSIX::SIGUSR1, $action);

#$SIG{USR1} = \&trap; #or install handler like this or "use sigtrap" whatever

sub trap {
    print "got signal.\n";
}

print "My pid is: $$\n";

while(sleep(1)) {

}

##</code><code>##

$ perl test.pl 
My pid is: 3188
$ kill -SIGUSR1 3188 
got signal.
$ kill -SIGUSR1 3188
got signal.
---> script is terminated

##</code><code>##

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>

void usr1handler(int signal);

int main(void) {
    signal(SIGUSR1, usr1handler);

    printf("My pid is: %d\n", getpid());

    for(;;) {
        sleep(1);
    }
    return 0;
}

void usr1handler(int signal) {
    printf("Got a signal: %d\n", signal);
}

##</code><code>##

$ ./test 
My pid is: 3188
$ kill -SIGUSR1 3318
Got a signal called: 10
$ kill -SIGUSR1 3318
Got a signal called: 10
$ kill -SIGUSR1 3318
Got a signal called: 10
$ kill -SIGUSR1 3318
Got a signal called: 10
... it never exits