You're using system to launch a shell to execute a shell command. Perl has no way no knowing if that command results in the launching of processes or what their PID would be.

The simplest solution would be to launch the program yourself. 

use IPC::Open3 qw( open3 );

sub launch_in_background {
   my ($prog, @args) = @_;
   open(local *CHILD_STDIN, '<', '/dev/null') or die $!;

   if (!@args) {
      # Make sure $prog isn't interpreted as a shell command.
      @args = ('-c', 'exec "$1"', 'dummy', $prog);
      $prog = '/bin/sh';
   }

   return open3('<&CHILD_STDIN', '>&STDOUT', '>&STDERR', $prog,@args);
}

my $pid1 = launch_in_background('prog', 'file1');
my $pid2 = launch_in_background('prog', 'file2');

[download]

Don't forget to reap your children or set SIGCHLD to IGNORE when you launch them.

IPC::Run3 or IPC::Run would provide more readable solutions.

In reply to Re: is it possible to get PID for a command running background? by ikegami
in thread is it possible to get PID for a command running background? by lightoverhead

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.