More readable, but doesn't work (or works differently :-)

• If the package name or 'all' is not in the environment variable, it returns 1 not the empty string!
• $dlv{__PACKAGE__} is the same as $dlv{"__PACKAGE__"} so it never checks %dlv for the package name
• /^(.*)=(\d+)$/ doesn't allow package names sans the "=" as the original did (the value should default to 1).
• If warnings are on and $ENV{DEBUG} is not define you get an undefined warning. My version does this too!

Have a test suite :-)

#! /usr/bin/perl 

use strict;
use warnings;
use Symbol qw(delete_package);
use Test::More;

my %variants = (

  adrianh_original => q{
    use constant DEBUG => do {
      my $p=__PACKAGE__; 
      ",$ENV{DEBUG}," =~ m/,($p|all)(=(.*?))?,/s && ($2 ? $3 : 1) 
    };
  },

  adrianh_no_warnings => q{
    use constant DEBUG => do {
      my ($package, $debug) = (__PACKAGE__, $ENV{DEBUG} || ''); 
      ",$debug," =~ m/,($package|all)(=(.*?))?,/s && ($2 ? $3 : 1)
    };
  },

  aristotle => q{
    use constant DEBUG => do {
      my %dlv = map /^(.*)=(\d+)$/, split /,/, $ENV{DEBUG};
      $dlv{__PACKAGE__} || $dlv{all} || 1;
    };
  },
);

my %test_values = (
  'Foo::Bar'         =>  1,
  'all'              =>  1,
  ''                 =>  '',
  'bar'              =>  '',
  undef              =>  '',
  'Foo::Bar=99,bar'  => 99,
  'bar,Foo::Bar=99', => 99,
  'all=99,bar'       => 99,
  'bar,all=99',      => 99,
);

plan tests => scalar(keys(%test_values)) * scalar(keys(%variants));

while (my ($variant, $test_constant) = each %variants) {
  while (my ($debug, $expected) = each %test_values ) {
    delete_package('Foo::Bar');
    if ($debug eq 'undef') {
      delete $ENV{'DEBUG'};
    } else {
      $ENV{'DEBUG'} = $debug;
    };
    eval qq{
      package Foo::Bar;
      use strict;
      use warnings;
      $test_constant;
      main::is(DEBUG, \$expected, "\$variant: \$debug");
    }; fail($@) if $@;
  };
};
[download]

Which produces

1..27
ok 1 - adrianh_original: 
ok 2 - adrianh_original: bar
ok 3 - adrianh_original: Foo::Bar=99,bar
Use of uninitialized value in concatenation (.) or string at (eval 7) 
+line 6.
ok 4 - adrianh_original: undef
ok 5 - adrianh_original: all=99,bar
ok 6 - adrianh_original: bar,Foo::Bar=99
ok 7 - adrianh_original: bar,all=99
ok 8 - adrianh_original: all
ok 9 - adrianh_original: Foo::Bar
not ok 10 - aristotle: 
#     Failed test ((eval 19) at line 11)
#          got: '1'
#     expected: ''
not ok 11 - aristotle: bar
#     Failed test ((eval 21) at line 11)
#          got: '1'
#     expected: ''
not ok 12 - aristotle: Foo::Bar=99,bar
#     Failed test ((eval 23) at line 11)
#          got: '1'
#     expected: '99'
Use of uninitialized value in split at (eval 25) line 7.
not ok 13 - aristotle: undef
#     Failed test ((eval 25) at line 11)
#          got: '1'
#     expected: ''
ok 14 - aristotle: all=99,bar
not ok 15 - aristotle: bar,Foo::Bar=99
#     Failed test ((eval 29) at line 11)
#          got: '1'
#     expected: '99'
ok 16 - aristotle: bar,all=99
ok 17 - aristotle: all
ok 18 - aristotle: Foo::Bar
ok 19 - adrianh_no_warnings: 
ok 20 - adrianh_no_warnings: bar
ok 21 - adrianh_no_warnings: Foo::Bar=99,bar
ok 22 - adrianh_no_warnings: undef
ok 23 - adrianh_no_warnings: all=99,bar
ok 24 - adrianh_no_warnings: bar,Foo::Bar=99
ok 25 - adrianh_no_warnings: bar,all=99
ok 26 - adrianh_no_warnings: all
ok 27 - adrianh_no_warnings: Foo::Bar
# Looks like you failed 5 tests of 27.
[download]