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use strict ;

my @cat = qw( cat ) ;
my ( $in_q, $out_q, $err_q ) ;

use IPC::Run qw( start pump finish timeout ) ;

# Incrementally read from / write to scalars.  Note that $in_q
# is a queue that is drained as it is used. $h is for "harness".
my $h = start \@cat, \$in_q, \$out_q, \$err_q, timeout( 10 ), debug => 1 ;

$in_q .= "some input\n" ;
pump $h until $out_q =~ /input\n/g ;

$in_q .= "some more input\n" ;
pump $h until $out_q =~ /\G.*more input\n/ ;

$in_q .= "some final input\n" ;
finish $h or die "cat returned $?" ;

warn $err_q if $err_q ; 
print $out_q ;         ## All of cat's output