perl hangs on write syscall

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I'm using below cmd to get data from remote server using SSH in perl :

 my $o = `ssh  -n -x -q -o StrictHostKeyChecking=no -o BatchMode=yes -
+o ConnectTimeout=5 host /script 2>/dev/null`;
<code>

<p> then, I process the $o and print to STDOUT .

Now, the problem here is, when the data returned by the ssh is huge, t
+hen, the perl program hangs forever.

I did truss of the process id, it shows output like 
</p>

<c>
 read(3, 0x001DF90C, 5120)                       = 5120
 ---chunk of data from remote host ---
 brk(0x001E2470)                                 = 0
 brk(0x001E2470)                                 = 0
 brk(0x001E4470)                                 = 0
 read(3, 0x001DF90C, 5120)                       = 5120
 ---chunk of data from remote host ---
 ---above continues until all data from rhost is read ---
 ---then starts write operation :
 write(4, 0x00229D64, 5120)                      = 5120
 ---writes chunk of data read from rhost
 ---does for some more chunks then hangs forever---
  write(4, 0x00229D64, 5120)      (sleeping...)
[download]

I also found out that the ssh process created by the perl program is not hanging.
Is there problem in storing huge data into a variable (memory) in perl ? Here's the ulimit of the system :

time(seconds)        unlimited
file(blocks)         unlimited
data(kbytes)         unlimited
stack(kbytes)        8192
coredump(blocks)     unlimited
nofiles(descriptors) 4096
vmemory(kbytes)      unlimited
[download]

Need your help in sorting the problem.

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Re: perl hangs on write syscall by salva (Canon) on May 16, 2015 at 11:34 UTC
Define huge. In any case, what's probably happening is that whatever is attached to the other side of file descriptor 4 (a pipe?) is not handling the data.	[reply]
Re: perl hangs on write syscall by RichardK (Parson) on May 16, 2015 at 11:42 UTC
How big is huge? Have you tried using open to read the output piped from your command and process the data a line at a time? `open (my $in,'-\|','ssh ...'); while (<$in>) { # do stuff }` [download]	[reply] [d/l]
Re: perl hangs on write syscall by pme (Monsignor) on May 16, 2015 at 11:14 UTC
Hi AM, What happens if you run your ssh command from shell redirecting the stdout into a file and it returns huge data?	[reply]
Re^2: perl hangs on write syscall by Anonymous Monk on May 16, 2015 at 11:26 UTC
Hi, it works doing ssh from shell and redirecting output to some file. Seems to be problem with writing of data read from the remote host.	[reply]