#  Somewhere early on, so that it can be used everywhere.
my $error_log;
...
#  I don't use 'or die' here because I use autodie.
open ( $error_log, '>', 'errors.log' );
...
#  Later, wherever it's necessary ..
print $error_log "$unique_identifier" .
  "$useful_variables $more useful variables\n";

##</code><code>##

system qq(echo  "$message" >> log.err);

##</code><code>##

print $error_log "$message\n";