my $pid = open3('<&STDIN', \*CHLD_OUT, \*CHLD_ERR, "some_shell_expression");
while(<CHLD_OUT>) {
    # do something with that command's output
}
my $err = <CHLD_ERR>; # may block - better use select here

waitpid $pid, 0;

if ( $? ) {
    warn $_[0]
    . ($? == -1) ? "failed to execute: $!\n"
    : ($? & 127) ? sprintf "child died with signal %d, %s coredump\n",
                ($? & 127),  ($? & 128) ? 'with' : 'without'
    : sprintf "child exited with value %d\n", $? >> 8;
}