scalar contextだと思ってた...

test.pl

#!/usr/bin/env perl
use strict;
use warnings;

my $aa = {
    aa => hoge(),
    bb => 'xxx',
};

sub hoge {
    return ;
}

use Data::Dumper;
warn Dumper($aa);
# => Odd number of elements in anonymous hash at test.pl line 5.
# $VAR1 = {
#           'aa' => 'bb',
#           'xxx' => undef
#         };