Here is a somewhat clunky delayed evaluation trick:

my %f;
%f=(One=>1,
    two=>sub{$f{One}}
);

# Flatten all the subs ..
ref $f{$_} eq 'CODE' and $f{$_}=$f{$_}->() for keys %f;

print Dumper \%f; # All populated
[download]

Output:

$VAR1 = {
          'two' => 1,
          'One' => 1
        };
[download]

Update: Verified that this works fine (i.e. 'One' and 'two' point to the same data) for HOH - i.e for:

my %f;
%f=(One=>{this=>'that'},
    two=>sub{$f{One}});
[download]