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

my @fookeys;

# select 'foo' from 'HoH' where 'foo' = 'bar2'
@fookeys = sort { $HoH{$a} -> {foo} eq 'bar2' } keys %HoH;

use Data::Dumper;

# fookeys is [ 1, 2 ]
print Dumper \@fookeys;

# try again
@fookeys = sort { $HoH{$a}{foo} eq 'bar2' } keys %HoH;

# fookeys is [ 1, 2 ]
print Dumper \@fookeys;