Two problems.

• cat still buffers its output when its STDOUT is connected to a terminal. Like Perl's default, it uses block buffering when its STDOUT isn't connected to a terminal and line buffering when it is. You'll have to send a newline to get it to react.

• You're program is too fast! cat never has a change to run. You need to wait for output from cat.

#!/usr/bin/perl -w
use strict;

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

my @tok_program = 'cat';

my ($TOK_IN, $TOK_OUT, $TOK_ERR);
my $TOK = start \@tok_program,
      '<',      \$TOK_IN,
      '1>pty>', \$TOK_OUT,
      '2>',     \$TOK_ERR
   or die "Error: $?;\n";

while (my $line = <STDIN> ) {
    # Send input.   
    $TOK_IN = $line;
    pump $TOK while length $TOK_IN;

    # Wait for output.
    pump $TOK while $TOK_OUT !~ /\n\z/;
    print "out: $TOK_OUT";
    $TOK_OUT = '';  
}
 
finish $TOK or die "returned $?";
[download]

• No reason to make STDIN a pty.
• Added missing call to finish.
• >pty> changed STDOUT and STDERR. Switched to 1>pty>.

Expect also uses a pty, and it might be better suited to control interactive application.

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