I first approached this by forming a "flattened" intermediate hash where the first and second level keys were joined with a NUL character. The keys of the new hash could then be sorted by the associated descending numeric values then used for the printf statement. This code

use strict;
use warnings;

my %hoh_test = (
   foo1 => { bar => -0.12697,        baz => -0.000398154, },
   foo2 => { bar => -4.0183e-05,     baz => 0, },
   foo3 => { bar => 9.966003977e-06, baz => 0.0001939, },
   );

my %flatter =
   map {
       my $k1 = $_;
       map {
           join( qq{\0}, $k1, $_ ), $hoh_test{ $k1 }->{ $_ }
       }
       keys %{ $hoh_test{ $k1 } };
   }
   keys %hoh_test;

my @sortedKeys =
   sort { $flatter{ $b } <=> $flatter{ $a } }
   keys %flatter;

printf qq{foo: %s, ba: %s, value: %s\n},
   split( m{\0}, $_ ), $flatter{ $_ }
   for @sortedKeys;
[download]

produces

foo: foo3, ba: baz, value: 0.0001939
foo: foo3, ba: bar, value: 9.966003977e-06
foo: foo2, ba: baz, value: 0
foo: foo2, ba: bar, value: -4.0183e-05
foo: foo1, ba: baz, value: -0.000398154
foo: foo1, ba: bar, value: -0.12697
[download]

This seemed a bit long-winded so I thought it might be possible to still use combined keys but mapping out array refs to perform an ST all in one fell swoop. This code

use strict;
use warnings;

my %hoh_test = (
   foo1 => { bar => -0.12697,        baz => -0.000398154, },
   foo2 => { bar => -4.0183e-05,     baz => 0, },
   foo3 => { bar => 9.966003977e-06, baz => 0.0001939, },
   );

printf qq{foo: %s, ba: %s, value: %s\n}, @{ $_ } for
   map  { [ split( m{\0}, $_->[ 0 ] ), $_->[ 1 ] ] }
   sort { $b->[ 1 ] <=> $a->[ 1 ] }
   map  {
        my $k1 = $_;
        map {
            [ join( qq{\0}, $k1, $_ ), $hoh_test{ $k1 }->{ $_ } ]
        }
        keys %{ $hoh_test{ $k1 } };
   }
   keys %hoh_test;
[download]

produces identical output. I hope this is of interest.

Update: Corrected typo, s/tha/the/