It may help to know what the const pragma does. It simply creates functions (subs) that are constant. A sub prototyped with () is defined to take no arguments and is then in-lined. Here's an evil method I wrote but never used in live code that may help you dig a little further:

sub const_override
{
    my $name = shift;
    my $pkg;

    if ($name =~ /^(.*)::([^:]+)$/)
    {
        $pkg = $1;
        $name = $2;
    }
    else
    {
        $pkg = caller;
    }

    no strict 'refs';

    if (!defined(*{"${pkg}::$name"}))
    {
        croak "A constant named '${pkg}::$name' has not been defined."
+;
    }

    carp "Constant override for ${pkg}::$name not in BEGIN block may f
+ail"
      if (defined($^S));

    local $SIG{__WARN__} = sub
    {
        warn @_ if (join(" ", @_) !~ /Constant subroutine .* redefined
+/);
    };

    if (@_ == 1)
    {
        my $scalar = $_[0];
        *{"${pkg}::$name"} = sub () { $scalar };
    }
    elsif (@_)
    {
        my @list = @_;
        *{"${pkg}::$name"} = sub () { @list };
    }
    else
    {
        my $set;
        *{"${pkg}::$name"} = sub () { };
    }
}
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