Reading non-blockingly / "awk has to be better for something."

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hail, monks.

I'm writing a GUI frontend to a command-line program, and am in need of a progress bar. The command-line program outputs things like this:

Initialising...\n
Progress: 0%\r
Progress: 1%\r
Progress: 2%\r
Progress: 3%\r
[download]

I have an I/O helper that can tell me when there is something waiting in the I/O buffer for me. It then calls my callback:

sub io_cb {
  my $fh = shift;
  local $/ = "\r";
  my $line = <$fh>; # blocks until there is a \r
  # update progress bar with $line
}
[download]

However, the kicker is that the command-line program's initialisation takes a while. Therefore, the sub will block the few seconds until the first percentage line is output, which leads to the UI freezing. ($line will then contain Initialising...\nProgress: 0%\r -- one line more than I hoped for)

It would be great if I could set $/ to qr/[\r\n]/, but the documentation tells me only awk supports that. Is there a way I could just read whatever is immediately available on $fh?

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