am I missing something?

Yes. I did not write about the system function, but about the system (as in "operating system"). I cited alarm in perlport: "Emulated using timers that must be explicitly polled whenever Perl wants to dispatch "safe signals" and therefore cannot interrupt blocking system calls. (Win32)" (Emphasis mine) I wrote " Replace sleep with something that blocks for 5 seconds outside perl (i.e. in the system) and alarm will no longer work."

Demo, using flock, a function using a system call that may block for a long time:

#!/usr/bin/perl

use strict;
use warnings;
use v5.10;
use Fcntl qw( LOCK_EX LOCK_UN );
use autodie qw( open flock close );

sub main
{
    # create a tempfile
    open my $h,'>>','tempfile.tmp';
    close $h;

    # start a background process that locks the tempfile for 10 second
+s
    if ($^O eq 'MSWin32') {
        system 1,$^X,$0,'locker';
    } else {
        my $pid=fork() // die "Can't fork: $!";
        unless ($pid) {
            exec $^X,$0,'locker' or die "Can't exec: $!";
        }
    }
    sleep 1; # wait one second for the helper process;

    open $h,'>>','tempfile.tmp';
    $@='';
    my $start=time();
    eval {
        local $SIG{'ALRM'}=sub { die "timeout" };
        say 'main: alarm 5';
        alarm(5);
        say 'main: flock LOCK_EX';
        flock($h,LOCK_EX);
        say 'main: alarm 0';
        alarm(0);
    };
    my $err=$@ || 'successfully locked';
    my $stop=time();
    say "main: $err";
    say 'main: ',$stop-$start,' seconds have passed';
    close $h;
    # allow locker() to finish before returning to command prompt
    ($^O eq 'MSWin32') ? sleep 1 : wait;
}

sub locker
{
    open my $h,'>>','tempfile.tmp';
    say 'locker: flock LOCK_EX';
    flock($h,LOCK_EX);
    say 'locker: locked';
    say 'locker: sleep 10';
    sleep 10;
    say 'locker: flock LOCK_UN';
    flock($h,LOCK_UN);
    say 'locker: unlocked';
    close $h;
}


@ARGV ? locker() : main();
[download]

How the demo works:

Without arguments, main() is invoked, main() creates a background process (using system(1,@args) on Windows, fork() and exec() on Linux), waits a second, then tries to lock a temp file with a classic timeout construct (eval, $SIG{'ALRM'}=sub { die }, alarm($timeout), system call, alarm(0)). Time for this is measured. The background process is the same script, but invoked with an argument, so that locker() is invoked instead of main(). Locker locks the temp file for 10 seconds, and because main() waits a second, it will succeed. In main(), the temp file is already locked, so flock() will block until locker() releases the lock OR the system call is interrupted by the ALRM signal caused by alarm().

On Linux, signals just work. flock() in main() is interrupted by the ARLM signal, the signal handler in $SIG{'ALRM'} is invoked, that terminales the eval block using die() after 5 seconds.

On Windows, signals are emulated. alarm($timeout) sets up a timer that must be polled. This is impossible during a blocking system call like flock(). So flock() blocks until it can aquire the lock after about 9 seconds (locker() waits 10 seconds after locking, main() waits 1 second before trying to lock, 9 seconds remain). $SIG{'ALRM'} is not invoked. alarm(0) disables the timer. The eval block terminates without an error.

Output on Linux:

>perl lockdemo.pl
locker: flock LOCK_EX
locker: locked
locker: sleep 10
main: alarm 5
main: flock LOCK_EX
main: timeout at lockdemo.pl line 30.

main: 5 seconds have passed
locker: flock LOCK_UN
locker: unlocked

>
[download]

Output on Windows:

H:\tmp>perl lockdemo.pl
locker: flock LOCK_EX
locker: locked
locker: sleep 10
main: alarm 5
main: flock LOCK_EX
locker: flock LOCK_UN
main: alarm 0
main: successfully locked
main: 10 seconds have passed
locker: unlocked

H:\tmp>
[download]

So why did alarm($timeout) "work" with system "sleep 10"?

system @args (unlike system 1,@args) waits for the sub-process to terminate in perl. (flock() waits in the operating system!) During that time, perl can poll the timer and thus can emulate the ARLM signal.

Alexander