The $SIG{CHLD} + sleep approach you took (instead of wait/waitpid) is interesting, since it saves you from using alarm. While an alarm would interrupt wait/waitpid, why use signals when they're not needed.

use POSIX qw( _exit );

if (!stat($png)) {
    my $child_code;

    local $SIG{CHLD} = sub {
        wait();  # Reap child
        $child_code = $?;
    };

    defined( my $pid = fork() )
        or die("Can't fork: $!\n");

    if (!$pid) {  # Child
        if (eval { create_thumbnail($pdf,$png); 1 }) {
            _exit(0);
        } else {
            # Especially deadly version of die()
            print STDERR $@;
            _exit($! || $?>>8 || 255);
        }
    }

    print "Waiting on $pid...\n";
    sleep($timeout);  # Interrupted by SIGCHLD since we have a handler
    if (defined($child_code)) {
        die("Child failure\n") if $child_code;
    } else {
        $SIG{CHLD} = 'DEFAULT';
        kill KILL => $pid;
        wait();  # Reap child
        die("Child timeout\n");
    }
}
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