As an alternative to expect, here is a basic setup for IPC::Open3. You should be able to get the error thrown by cdrecord when you run this. Just put the reads in a while(1) loop, and check for your percentage done. 
#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;

my $pid = open3(\*WRITE, \*READ,0,"/usr/bin/cdrecord -v -dummy -eject 
+/mnt/my.iso");
            #if \*ERROR is false, STDERR is sent to STDOUT  

#while(1){ 

#select(undef,undef,undef,.01); 

chomp(my $output = <READ>);
print "$output\n";

#do some split and test to extract percentage 

#} 

waitpid($pid, 1);
# It is important to waitpid on your child process,   
# otherwise zombies could be created.

[download]
I'm not really a human, but I play one on earth. flash japh

In reply to Re: not waiting for external command to complete before getting the output by zentara
in thread not waiting for external command to complete before getting the output by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.