use bimodal_constant FOO => "BAR";

print "\$FOO is $FOO\n" if FOO;

##</code><code>##

package bimodal_constant;

sub import {
    my ($class, $key, $value) = @_;
    my $caller = caller();

    # constant scalar
    eval " *$caller\::$key = \\\"$value\" ";
    # constant sub
    eval " *$caller\::$key = sub () { \"$value\" } ";
}

1;