Because in this case you want a literal "$9", you don't want it to be expanded by Perl. An example that you can experiment with: 
% date
Wed Sep  4 10:41:55 BST 2013
% date | awk '{ print $2 }'
Sep
% perl -de0

Loading DB routines from perl5db.pl version 1.32
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):    0
  DB<1> $cmd = qq[date | awk '{ print $2 }']

  DB<2> p $cmd
date | awk '{ print  }'
  DB<3> p qx/$cmd/
Wed Sep  4 10:47:41 BST 2013

  DB<4> $cmd = qq[date | awk '{ print \$2 }']

  DB<5> p $cmd
date | awk '{ print $2 }'
  DB<6> p qx/$cmd/
Sep

  DB<7> q
%

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In reply to Re^5: how to use perl variable in system command to run on unix by Arunbear
in thread how to use perl variable in system command to run on unix by tousifp

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