No scoped constant pragma (yet).

Since constants are implemented (still) as subroutines subject to inlining which occupy a CODE slot in the symbol table, there's no such thing until lexical globs are implemented (you wrote the primary patch for that).

Just trying reveals:

#!/usr/local/bin/perl5.10.0

use constant FOO => 'quux';
{
   use constant FOO => 'bar';
}
my $foo = FOO;
print $foo,"\n";
__END__
Constant subroutine main::FOO redefined at /usr/local/lib/perl5/5.10.0
+/constant.pm line 115.
bar
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