use IPC::Open3 qw( open3 );

sub run_child { 
   my ($widget) = @_;

   my $pid = open3(
      my $to_child,   # Autovivified when false.
      my $fr_child,   # Autovivified when false.
      undef,          # Same as $fr_child when false.
      $somecommand,
      @args
   );

   while (<$fr_child>) {
      my $end_is_visible = ( $yview == 1.0 );
      $text->insert('end', $_);
      $w->see('end') if $end_is_visible;
   }

   waitpid($pid, 0);
}