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use IPC::Open2;

...
use FileHandle;
my($rdr, $wtr) = (FileHandle->new, FileHandle->new);
$pid = open2($rdr, $wtr, $cmd_with_args);

use IPC::Open2;
use Symbol;
...
     $line = <$RDR>;         # read the output of bc(1)
     print STDOUT "$line";   # send the output to the user
}

$pid = open2($RDR, $WTR, 'sort');

...
while (<$RDR>) {     # now read the output of sort(1)
     print STDOUT "$_";
}