In

use constant {
  zed     => 0,
  one     => 1,
  repos   => (qw(oss non-oss debug)),
  two     => 2,
};
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the

{
  zed     => 0,
  one     => 1,
  repos   => (qw(oss non-oss debug)),
  two     => 2,
}
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part is actually an anonymous hash constructor that passes its value (a hash reference) to the constant pragma for further processing. So this hash has to be something sensible, and

c:\@Work\Perl\monks>perl -wMstrict -MData::Dumper -le
"print Dumper {
   zed     => 0,
   one     => 1,
   repos   => (qw(oss non-oss debug)),
   two     => 2,
 };
"
$VAR1 = {
          'non-oss' => 'debug',
          'repos' => 'oss',
          'one' => 1,
          'zed' => 0,
          'two' => 2
        };
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ain't it. As you can see,  'non-oss' ends up as a key with  'debug' as its value because  qw(oss non-oss debug) is just a flattened list within the constructor.

So no, you can't use the  { ... } version of constant as you initially wished because a valid hash must be built first.

The  use constant repos2 => qw(oss non-oss); version works because the  qw() expression is what the  repos2 function created by constant is built to return. (Update: constant distinguishes between this syntax and the  { ... } hash syntax by virtue of the type of the first argument passed to it.

c:\@Work\Perl\monks>perl -wMstrict -le
"use constant [ qw(a b c d) ];
"
Invalid reference type 'ARRAY' not 'HASH' at -e line 1
BEGIN failed--compilation aborted at -e line 1.

c:\@Work\Perl\monks>perl -wMstrict -le
"use constant qw(A b c d);
 print A;
"
bcd
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)