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>
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
Is that more in line with what you really wanted?