This is slightly tangential to your problem but I wonder whether you might be able to use a single HoH structure in your problem rather than two separate hashes.

use strict;
use warnings;

use List::Util qw{ max };
use Data::Dumper;

my %hashA = (
   11.5  => {
      1723478 => 237,
      1734789 => 114,
      1798761 => 147,
      },
   11.12 => {
      1700123 => 123,
      },
   11.01 => {
      1780345 => 456,
      },
   );

print Data::Dumper->Dumpxs( [ \ %hashA ], [ qw{ *hashA } ] );

delete $hashA{ max keys %hashA }->{ q{1734789} };

print Data::Dumper->Dumpxs( [ \ %hashA ], [ qw{ *hashA } ] );
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The output.

%hashA = (
           '11.01' => {
                        '1780345' => 456
                      },
           '11.12' => {
                        '1700123' => 123
                      },
           '11.5' => {
                       '1734789' => 114,
                       '1723478' => 237,
                       '1798761' => 147
                     }
         );
%hashA = (
           '11.01' => {
                        '1780345' => 456
                      },
           '11.12' => {
                        '1700123' => 123
                      },
           '11.5' => {
                       '1723478' => 237,
                       '1798761' => 147
                     }
         );
[download]

I hope this is useful.