it means we get the output from the command and we use that output as a filename.right

(Incidentally: s/$/?/)

No, it's not right. It means that a process is forked for that command and that the output of that command, i.e. what that it will inject into STDOUT is returned to you when you read from the opened filehandle, as if it were the content of an actual file.

#!/usr/bin/perl

my $pid = $$;

open (T,"top -d 1 -p $pid b |" ) or die $!;
$|=1;
while(<T>){
  $output = $_;
  $output =~ tr/\n//;
#print $output; 

  my @words=split(/\s+/,$output);
print join "\n",@words,"**************\n";

#  if($words[0] =~ /$pid/){print "$words[4]\n"} 
}
[download]

Here is a super-simple piped input, using a perl script.

#the Main script to run

#!/usr/bin/perl -w
$|++;
open( H, " | other_script") or die "$!\n";
print H "hi\n";
sleep 1; #let other script print before dieing  
__END__
#####################################
#the other_script

#!/usr/bin/perl
$|++;
my $in = <>;
print "$in\n"
__END__
[download]

open(handle,"type d:/help.txt |");
[download]

Enjoy,
Mickey