sub heavy_export {
    # ...
    my($pkg, $callpkg, @imports) = @_;
    my($type, $sym, $cache_is_current, $oops);
    my($exports, $export_cache) = (\@{"${pkg}::EXPORT"},
                                   $Exporter::Cache{$pkg} ||= {});

    # ...

    my($fail, $fail_cache) = (\@{"${pkg}::EXPORT_FAIL"},
                              $Exporter::FailCache{$pkg} ||= {});

    if (@$fail) {
	if (!%$fail_cache) {
	    # Build cache of symbols. Optimise the lookup by adding
	    # barewords twice... both with and without a leading &.
	    # (Technique could be applied to $export_cache at cost of memory)
	    my @expanded = map { /^\w/ ? ($_, '&'.$_) : $_ } @$fail;
	    warn "${pkg}::EXPORT_FAIL cached: @expanded" if $Exporter::Verbose;
	    @{$fail_cache}{@expanded} = (1) x @expanded;
	}
	my @failed;
        # ...
    }

    # ...
}