If you checked the return status of your open(PL, ...) call, I bet you would find out that it failed. Since there is no pipe symbol at the end of the file name, you are actually asking perl to read a file with a lot of shell metacharacters in it.

An easier way is just to use system() to directly invoke the command:

system("grep 'packet loss' ... > /home/.../$TARGET_HOST.PL.log");
[download]

If you wanted to open a pipe from the command, you would need to terminate it with a pipe symbol, e.g.

open(PL, "grep ... | ... |")
  or die "unable to open pipe from grep command: $!";
while (<PL>) { ... }
close(PL);
[download]