# OK to use blocking calls, the data will be there
# don't read past the "DONE", wait for next file event
# otherwise this will "hang". 

my $msg;
while (my $line = <$child_fh>)
{
   $msg =. $line;
   last if $line =~ /^:DONE:\s*$/;
}

if ($msg !~ /^:MSG:/) {framing error!}
do what you need to do...
$msg = undef;