If you put your one-liner into a file, then run that under the debugger, the warning is emitted.

$ perl -d it.pl
Default die handler restored.

Loading DB routines from perl5db.pl version 1.07
Editor support available.

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

main::(it.pl:6):        my $bar = { foo => () };
  DB<1> s
Odd number of elements in hash assignment at it.pl line 6.
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.  
  DB<1>
[download]

I have no explanation. I don't tend to use the debugger very often, and when I do, I wouldn't use it for just one line of code. I do sometimes use B::Deparse to deconstruct a snippet of code:

perl -MO=Deparse it.pl
my $bar = {'foo', ()};
it.pl syntax OK
[download]

Is that more in line with what you really wanted?