Using the memory gobbling technique by Corion here, let's do some testing. I divided by 2 to reflect the actual memory consumption desired.

My virtual CentOS 7 machine has 4 GB of RAM allocated to it. This creates a 3 GB scalar key-value pair to consume 75%. The system command called by the main process fails, no ls output on the 2nd time. Calling syscmd succeeds due to the background worker spun early.

syscmd

use strict;
use warnings;

use MCE::Child;
use MCE::Channel;

my $chnl = MCE::Channel->new( impl => 'Simple' );

# spin up worker early before creating big hash
mce_child {
    local $SIG{__WARN__} = sub {};
    while ( my ($cmd, @args) = $chnl->recv ) {
        local ($?, $!);
        system($cmd, @args);
        $chnl->send2($?, $!);
    }
};

sub syscmd {
    my $cmd = shift;
    return unless $cmd;

    $chnl->send($cmd, @_);
    my ($status, $errmsg) = $chnl->recv2;
    
    if ($status == -1) {
        print "SYSTEM: failed to execute ($cmd): $errmsg\n";
    }
    elsif ($status & 127) {
        printf "SYSTEM: $cmd died with signal %s, %s coredump\n",
            ($status & 127), ($status & 128) ? 'with' : 'without';
    }
    else {
        printf "SYSTEM: $cmd exited with status %d\n", $status >> 8;
    }
}

# My CentOS VM has 4 GB of RAM

# create big hash
my $memory_eaten = 3 * 1024*1024*1024 / 2; # 3 GB, adjust to fit
my %memory_eater = (
    foo => scalar( ' ' x $memory_eaten ),
);

# pass command and optionally args
syscmd('ls');  # this one works; see status that it succeeded
system('ls');  # this one fails; no ls output the 2nd time

# attempt to run a command not found
syscmd('something');

# sleep for 2 seconds
syscmd('sleep', '2');

# busy loop, see top output in another terminal
# notice the memory consumption (i.e. RES)
# press Ctrl-C to exit or let it finish
1 for 1..3e8;

# notify no more work, then reap worker
$chnl->end;
MCE::Child->waitall;
[download]

output:

ls output from syscmd
SYSTEM: ls exited with status 0
SYSTEM: failed to execute (something): No such file or directory
SYSTEM: sleep exited with status 0
[download]

Regards, Mario