The third field of ps -ef is the parent pid, did you mean that? Also, you can avoid the extra grep -v with a simple RE trick:

$module = "xyz ab[c]";
$pid = `ps -aef|grep  \"$module\"|cut -d ' ' -f3`;
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Note the [ ] around one of the characters.

Back to perl, here is a simple one-liner (OK, three lines) which only creates one child process:

my $module = 'xyz abc';
my @ppids = map {(split)[2]} grep /$module/,`ps -ef`;
print "@ppids\n";
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If you really wanted the pid rather than the ppid then change the [2] to [1].
Notice that I am using an array, since there might be more than one process which matches.