$ perl -MScalar::Util=weaken -e'weaken(\my $var)'

$ sudo mv /usr/lib/perl5/5.8.8/x86_64-linux-thread-multi/auto/List/Util/Util.so /usr/lib/perl5/5.8.8/x86_64-linux-thread-multi/auto/List/Util/Util.so_hide

$ perl -MScalar::Util=weaken -e'weaken(\my $var)'
Weak references are not implemented in the version of perl at -e line 0
BEGIN failed--compilation aborted.