I feel a little reticent about giving further advice as you seem to be doing pretty much the opposite of what I'm saying. Regarding installing Term::ReadLine::Perl, I suggested trying this "In the absence of other solutions": you chose to do this first. Regarding Perlbrew/MacPorts/ActiveState, I said "You may want to consider these options before adding or updating modules used by your system Perl.": you chose to add/update Term::ReadLine::Perl on your system Perl. However, it's your computer and you should be free to break it in whatever interesting ways you see fit.

I have a number of Perls installed - so you can "go that route":

System Perl

$ /usr/bin/perl -v

This is perl 5, version 12, subversion 3 (v5.12.3) built for darwin-th
+read-multi-2level
(with 2 registered patches, see perl -V for more detail)
...
[download]

Perlbrew Perl

$ /usr/bin/env perl -v

This is perl 5, version 14, subversion 2 (v5.14.2) built for darwin-th
+read-multi-2level
...
[download]

MacPorts Perl

$ /opt/local/bin/perl -v

This is perl 5, version 12, subversion 3 (v5.12.3) built for darwin-mu
+lti-2level
...
[download]

I tried your code above (presumably /Users/mike/readline.pl) with each of these three Perls. I was unable to reproduce your problem. I received no error or warning messages. I entered the same text for all of them:

$ pm_readline_perl_mac.pl
Enter: 
> 1+2
$ pm_readline_perl_mac.pl
Enter: 
> blah blah
$
[download]

And finally, just for completeness, I haven't set PERL_RL:

$ echo $PERL_RL

$
[download]