my %HoH =
( 1 => { foo => 'bar',
          baz => 'bletch' },
  2 => { foo => 'bar2',
          baz => 'bletch2' },
);

my @fookeys = grep { $HoH{$_}{foo} eq 'bar2' } keys %HoH;
# @fookeys is now 2