Not sure if I would recommend Readonly over constant — at least some people have been bitten by its subtle peculiarities. As an example, see this thread.

BTW, there's also Scalar::Readonly, which is directly manipulating the readonly-flag of scalars (instead of working with tied magic)...

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Update:  (...it was only a matter of time until someone would post a benchmark, so... :)

use strict;
use warnings;
use Benchmark 'cmpthese';

use Readonly;
use Scalar::Readonly ':all';
use constant NL => "\012";
Readonly my $NL => "\012";
my $nl = "\012";  readonly_on($nl);
my $Nl = "\012";

cmpthese 2000000 => {
    constant  => sub { my $s = "abc".NL."def".NL."ghi"; },
    constant2 => sub { my $s = "abc${\NL}def${\NL}ghi"; },
    Readonly  => sub { my $s = "abc${NL}def${NL}ghi";   },
    ScalarRO  => sub { my $s = "abc${nl}def${nl}ghi";   },
    scalarRW  => sub { my $s = "abc${Nl}def${Nl}ghi";   },
    direct    => sub { my $s = "abc\012def\012ghi";     },
};

__END__

               Rate  Readonly constant2  scalarRW  ScalarRO  constant 
+   direct
Readonly    77851/s        --      -84%      -85%      -85%      -96% 
+     -96%
constant2  484262/s      522%        --       -5%       -5%      -75% 
+     -75%
scalarRW   510204/s      555%        5%        --       -0%      -74% 
+     -74%
ScalarRO   511509/s      557%        6%        0%        --      -74% 
+     -74%
constant  1960784/s     2419%      305%      284%      283%        -- 
+      -0%
direct    1960784/s     2419%      305%      284%      283%        0% 
+       --
[download]

(as running the benchmark multiple times suggests, differences up to around 5% are insignificant)

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BTW, I think I remember PBP somewhere saying it's not meant to be treated as a bible...