#!/usr/bin/perl

use warnings FATAL => qw( all );
use strict;

use POSIX qw( :signal_h );

$SIG{ALRM} = sub { print "SIGARLM: main handler\n" };

sub f {
        my $sigset = POSIX::SigSet->new;
        my $blockset = POSIX::SigSet->new( SIGALRM );

        sigprocmask(SIG_BLOCK, $blockset, $sigset );
        $SIG{ALRM} = sub { print "SIGARLM\n" };
        sigprocmask(SIG_SETMASK, $sigset );
        # do something useful here
        sigprocmask(SIG_BLOCK, $blockset, $sigset );
        $SIG{ALRM} = sub { print "SIGARLM: main handler\n" };
        sigprocmask(SIG_SETMASK, $sigset );
}

if (fork) {
        # parent
        f while 1;
} else {
        # child
        sleep 1;
        print "child starting\n";
        1 while kill ALRM => getppid;
}