Hi, that message seems to refer to an issue with your Mozilla-based browser. I don't know how this comes into play in your program; I did look at the source of feat a little bit and saw that it outputs HTML, so maybe related?

Anyway I wanted to point you to one of many ways to capture all the output from your external command, Capture::Tiny. You could use something like this to examine the output and perhaps see where the source of the message is:

use Capture::Tiny ':all';

my ($stdout, $stderr, $exit) = capture {
  system( $program, $outfile );
};

print sprintf('Stderr: >%s< Stdout: >%s<', $stderr, $stdout);

if ( $exit != 0 ) {
    die sprintf('%s exited with status %s.', $program, $exit;
}
[download]

Hope this helps!