When I executed the following perl code on the command line with a -e option,
$s=q(top -bn1 -p $PPID); system $s; $v="\0"x(2*1024**3); print"\n"x3; system $s
Its output may look like this,
$ perl -e '$s=q(top -bn1 -p $PPID); system $s; $v="\0"x(2*1024**3); print"\n"x3;system $s'
top - 20:11:27 up 5 days, 4:10, 1 user, load average: 0.24, 0.10, 0.08
Tasks: 1 total, 0 running, 1 sleeping, 0 stopped, 0 zombie
%Cpu(s): 3.3 us, 0.0 sy, 0.0 ni, 96.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 7937.6 total, 3065.8 free, 4144.2 used, 727.6 buff/cache
MiB Swap: 2048.0 total, 87.8 free, 1960.2 used. 3433.7 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
4133828 sam 20 0 2110500 2.0g 4712 S 0.0 25.9 0:02.35 perl
top - 20:11:31 up 5 days, 4:10, 1 user, load average: 0.30, 0.11, 0.09
Tasks: 1 total, 0 running, 1 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 7937.6 total, 1029.3 free, 6180.7 used, 727.6 buff/cache
MiB Swap: 2048.0 total, 87.8 free, 1960.2 used. 1397.2 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
4133828 sam 20 0 4207656 4.0g 4712 S 0.0 51.7 0:06.17 perl
I found it first used about 2.0g in RES before assignment and then 4.0g.
If executing a simple top command in perl,
$ perl -e 'system "top -bn1 -p $PPID"'
top: -p requires argument
$ perl -e 'system "top -bn1 -p \$PPID"'
top - 20:27:18 up 5 days, 4:26, 1 user, load average: 0.13, 0.17, 0.13
Tasks: 1 total, 0 running, 1 sleeping, 0 stopped, 0 zombie
%Cpu(s): 1.6 us, 0.0 sy, 0.0 ni, 98.4 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 7937.6 total, 4600.3 free, 2081.7 used, 1255.6 buff/cache
MiB Swap: 2048.0 total, 89.0 free, 1959.0 used. 5496.1 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
4142493 sam 20 0 13344 3664 3384 S 0.0 0.0 0:00.00 perl
it used 3664 kB (perl -E 'say 3664/1024' => 3.578125) in RES.
Other
$ perl --version
This is perl 5, version 30, subversion 0 (v5.30.0) built for x86_64-linux-gnu-thread-multi
(with 60 registered patches, see perl -V for more detail)
Copyright 1987-2019, Larry Wall
...
Due to constant folding, "\0"x(2*1024**3) creates a 2 GiB string at compile-time.
$ perl -MO=Concise,-exec -e'$v = "\0" x 6;'
1 <0> enter v
2 <;> nextstate(main 1 -e:1) v:{
3 <$> const[PV "\000\000\000\000\000\000"] s/FOLD
4 <#> gvsv[*v] s
5 <2> sassign vKS/2
6 <@> leave[1 ref] vKP/REFC
-e syntax OK
(I'm using shorter strings in my examples, but I confirmed the same behaviour happens for "\0"x(2*1024**2).)
Assigning a string normally uses no memory thanks to the Copy-on-Write ("COW") mechanism.
$ perl -MDevel::Peek -e'$v = "abcdef"; Dump($v);'
SV = PV(0x55cb5eec3f20) at 0x55cb5eef32e0
REFCNT = 1
FLAGS = (POK,IsCOW,pPOK) <--- Copy avoided thanks to COW.
PV = 0x55cb5ef3aaf0 "abcdef"\0
CUR = 6
LEN = 16
COW_REFCNT = 1
However, the COW mechanism doesn't appear to be used for folded constants.
$ perl -MDevel::Peek -e'$v = "abc"."def"; Dump($v);'
SV = PV(0x5632e85d3f20) at 0x5632e86030d0
REFCNT = 1
FLAGS = (POK,pPOK) <--- String buffer isn't shared.
PV = 0x5632e85ef540 "abcdef"\0
CUR = 6
LEN = 16
$ perl -MDevel::Peek -e'$v = "\0" x 6; Dump($v);'
SV = PV(0x555a533d3f20) at 0x555a534031e0
REFCNT = 1
FLAGS = (POK,pPOK) <--- String buffer isn't shared.
PV = 0x555a533ef540 "\x00\x00\x00\x00\x00\x00"\0
CUR = 6
LEN = 16
Therefore, $v = "\0"x(2*1024**3); creates a copy of the string.
This is a bug that has now been fixed in upcoming 5.42.