Something that will let you use scalars but will prevent them being modified:

my $const = \"Don't touch me!";
my $num   = \5.6;
local *const2 = \"Another string";
local *num2   = \3.14159;
print "[$$const][$$num][$const2][$num2]\n";

### try and modify it
eval{ $num2 = 3; };
print $@;
print "$num2\n";
[download]

The first ones are not as pretty as dollar variables, and the second have a slight overhead of local, but I think that should give you some nice unmodifiable variables.

UPDATE:
Benchmarks are always good. Read em and weep:

#!/usr/bin/perl -w

use Benchmark qw(timethese cmpthese countit timestr);

use constant CONST_MOD => 3;
sub CONST_SUB () { 3 };
my    $const_my = 3;
local $const_local = 3;
my    $const_ref = \3;
local *const_alias = \3;

print "mod[".CONST_MOD."]sub[".CONST_SUB."]\n"
  ."my[$const_my]local[$const_local]\n"
  ."ref[$$const_ref]alias[$const_alias]\n";

cmpthese (1_000_000,
            {
              CMod => sub { my $t = CONST_MOD },
              CSub => sub { my $t = CONST_SUB },
              CMy  => sub { my $t = $const_my },
              CLocal => sub { my $t = $const_local },
              CRef   => sub { my $t = $$const_ref },
              CAlias => sub { my $t = $const_alias },
            });
[download]

Produces the following:

mod[3]sub[3]
my[3]local[3]
ref[3]alias[3]
Benchmark: timing 1000000 iterations of CAlias, CLocal, CMod, CMy, CRe
+f, CSub...
    CAlias:  0 wallclock secs ( 0.79 usr +  0.00 sys =  0.79 CPU) @ 12
+65822.78/s (n=1000000)
    CLocal:  0 wallclock secs ( 0.80 usr +  0.00 sys =  0.80 CPU) @ 12
+50000.00/s (n=1000000)
      CMod:  2 wallclock secs ( 0.80 usr +  0.00 sys =  0.80 CPU) @ 12
+50000.00/s (n=1000000)
       CMy:  1 wallclock secs ( 0.75 usr +  0.00 sys =  0.75 CPU) @ 13
+33333.33/s (n=1000000)
      CRef:  0 wallclock secs ( 1.03 usr +  0.00 sys =  1.03 CPU) @ 97
+0873.79/s (n=1000000)
      CSub:  0 wallclock secs ( 0.81 usr +  0.01 sys =  0.82 CPU) @ 12
+19512.20/s (n=1000000)
            Rate   CRef   CSub CLocal   CMod CAlias    CMy
CRef    970874/s     --   -20%   -22%   -22%   -23%   -27%
CSub   1219512/s    26%     --    -2%    -2%    -4%    -9%
CLocal 1250000/s    29%     3%     --    -0%    -1%    -6%
CMod   1250000/s    29%     3%     0%     --    -1%    -6%
CAlias 1265823/s    30%     4%     1%     1%     --    -5%
CMy    1333333/s    37%     9%     7%     7%     5%     --
[download]

The only noticable differences are the my and the ref. The reference takes too long, the "my" is a little quicker but it is modifiable. All of the others are neck a neck, so, I'd vote for local *var=\23;

my @a=qw(random brilliant braindead); print $a[rand(@a)];

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