Getting warning message while running the following code

Dear list

while I am running the following code I am getting a warning message. the
code is as follows

1 use strict;
2 use warnings;
3 use Cwd 'abs_path';
4 my $dir = "/";
5 chdir($dir);
6 #my $filename="$dir/processinfo.txt";
7 #`ps -U oraoneload_beta -u oraoneload_beta u | grep perl | grep -v grep
$filename`;
8 open(FH,"ps -U oraoneload_beta -u oraoneload_beta u | grep perl | grep
-v grep|") or "can't open : $!";
9 #open(FH,"$filename");
10 my @process = <FH>;
11 close(FH);
12 chomp(@process);
13 pop(@process);
14 foreach my $el(@process)
15 {
16 my $cmd =(split /\s+/,$el)[12];
17 my ($a,$b) = split(/\//,$cmd);
18 my $path=abs_path($b);
20 print $path."\n";
21 }
22

The warning message is as Useless use of string in void context at
processcheck.pl line 8. But while I am commenting the line 8 and
uncommenting the the line 6, 7 and 9 and then running this script I am not
getting any warning message. Where I am making the mistake?

Thanks & Regards in advance
Anirban Adhikary.