# (save STDIN/STDOUT here)
open STDIN, "<&".fileno(*SOCK)
    or  die "Can't dup SOCK to STDIN: $!$/";
open STDOUT, ">&".fileno(*SOCK)
    or  die "Can't dup SOCK to STDOUT: $!$/";
system( ... )
   and  ...;
# (restore STDIN/STDOUT here)