our @GLOBAL_SYMBOLS; # these are not package constants, but Perl artif
+acts
BEGIN {
    @GLOBAL_SYMBOLS = qw(BEGIN EXPORT);
}
[download]

sub is_constant {
    my $k = shift;
   return $k =~ /^[A-Z]+$/ 
       && !grep { $_ eq $k } @GLOBAL_SYMBOLS
}
[download]

Bah, use a hash, Luke! Use a hash!

our %GLOBAL_SYMBOL; # these are not package constants, but Perl artifa
+cts
BEGIN {
    %GLOBAL_SYMBOL = map { $_ => 1 } qw(BEGIN EXPORT ISA EXPORT_OK GLO
+BAL_SYMBOL);  
}
[download]

sub is_constant {
    my $k = shift;
   return $k =~ /^[A-Z_0-9]+$/
       && !$GLOBAL_SYMBOL{$k};
}
[download]

BTW did you forget to exclude @GLOBAL_SYMBOLS, by any chance? And, why not use a lexical for it?

BTW I think this functionality is generally interesting enough to deserve its own module on CPAN. Let's call it ExportAll, or something else like that. And, as the helper subs would reside in that module, they wouldn't be exported themselves.

Update: Oh no, I see you only support only upper case letters only names (which would exclude @GLOBAL_SYMBOLS). Which seems too restrictive to my taste.