`...` is just short for qx`...`, which is short for readpipe(qq`...`).

$ perl -MO=Concise,-exec -e'`...`'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <$> const[PV "..."] s
4  <1> backtick[t1] vK
5  <@> leave[1 ref] vKP/REFC
-e syntax OK

$ perl -MO=Concise,-exec -e'qx`...`'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <$> const[PV "..."] s
4  <1> backtick[t1] vK
5  <@> leave[1 ref] vKP/REFC
-e syntax OK

$ perl -MO=Concise,-exec -e'readpipe(qq`...`)'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <$> const[PV "..."] s
4  <1> backtick[t2] vK/1
5  <@> leave[1 ref] vKP/REFC
-e syntax OK
[download]

This is by no means new; this has been the case for at least 18 years. (The oldest documentation I could find is perl5.005_53 from Oct 31st, 1998.)

Found an example, where the daemonization closes STDOUT and STDIN in order to not create files. This could confuse the child process (which has no stdout). So either, maybe this works:

use Daemon::Simple;

open(SAVEIN, "<&STDIN");
open(SAVEOUT, "<&STDOUT");

Daemon::Simple::init("daemon");

open(STDIN, "<&SAVEIN");
open(STDOUT, "<&SAVEOUT");

print qx("/external/command");

close(STDIN);
close(STDOUT);
[download]

or you will have to redirect your output to a file

qx("/external/command > /tmp/file")

and read it back using open(), diamond and close()