REAP called for external programs called with backtricks, however it seems it does not affect anything in your case.

my $queue = [];
sub REAP {
    my $kidpid;
    print STDERR "HANDLER\n";
    while(($kidpid = waitpid(-1,WNOHANG)) > 0) {
        print STDERR "reaped $kidpid\n";
        push @$queue, [ $kidpid, $? ];
    }
    print STDERR "/HANDLER\n";
}
$SIG{'CHLD'} = \&REAP;

`echo 1`;

my $pid = fork();
print STDERR "FORK CHILD $pid\n" if $pid;
exit unless ($pid);

print STDERR "before sleep\n";
sleep 2;
print STDERR "after sleep\n";
print STDERR "DONE\n";
__END__


HANDLER
/HANDLER
FORK CHILD 29104
before sleep
HANDLER
reaped 29104
/HANDLER
after sleep
DONE
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