$ perl -Mwarnings -Mstrict -e 'my $bar = { foo => () }'
Odd number of elements in anonymous hash at -e line 1.
##</code><code>##

$ perl -Mwarnings -Mstrict -de 1

Loading DB routines from perl5db.pl version 1.27
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   1
DB<1> x { foo => () }
0  HASH(0x9c5b8f8)
   'foo' => undef
DB<2> q