But alarm sends a signal to the process that called alarm, not the child process, right?

Unless the process that called alarm is the process that created child, and by killing the process you kill the child. Try alternating 5 and 2 in the script I have whipped up below (make sure you read the warning, if file xyz is written then child has terminated OK, if not it was timed out):

#!/usr/bin/env perl

# for https://perlmonks.org/?node_id=1231710
# author: bliako
# 27/03/2019
# WARNING: it writes and deletes file 'xyz' in local dir

use strict;
use warnings;

sub spawner {
    my ($cmd, $timeout) = @_;
    print "spawner starting.\n";
    my $pid = fork();
    if( ! $pid ){
        print "spawner in child.\n";
        eval {
            local $SIG{ALRM} = sub { die "alarm blah blah" };
            alarm $timeout;
            print "starting system command.\n";
            `$cmd`;
            print "system command ended.\n";
            alarm 0;
        };
        if( $@ =~ /^alarm blah blah/ ){ die "spawner: child timed out"
+ }
        elsif( $@ ){ die "spawner: something wrong with eval block whi
+ch spawns '$cmd'" }
        exit 0;
    }
    print "spawner ending.\n";
}

spawner('rm -f xyz; sleep 5; touch xyz', 2);

wait();

if( -e 'xyz' ){ print "command seems to have terminated on its own wil
+l.\n"; }
else { print "command has timed out.\n"; }
[download]

bw, bliako

"But alarm sends a signal to the process that called alarm, not the child process, right? Even if it sent it to the child, the child process would need a signal handler for that process."

Although I've caused all manner of consternation with doing process stuff with Windows (including killing off sub-procs), I've found that Perl is weird when talking processes/threads with that OS. "Processes", I've managed (mostly, but with difficulty), but threads, I'd definitely need advice from our experts.

Perhaps you could lay out at least some pseudo-code (or minimal working/broken problematic example) of what you've got, so the experts can see if they can help sort out a solution?