foreach my $key ( keys %hash ) { 
  while ( my ( $subkey, $value ) = each %{ $hash{$key} } ) { 
    $hash{ $subkey }{ $key } = $value;
  }
  delete $hash{ $key };
}

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
use feature qw/ say /;
use Data::Dumper;

my %hash;
$hash{'keyA'}{'keyB'} = '1234';
$hash{'keyC'}{'keyB'} = '5678';

say Dumper \%hash;

foreach my $key ( keys %hash ) { 
  while ( my ( $subkey, $value ) = each %{ $hash{$key} } ) { 
    $hash{ $subkey }{ $key } = $value;
  }
  delete $hash{ $key };
}

say Dumper \%hash;

__END__

##</code><code>##

$VAR1 = {
          'keyA' => {
                      'keyB' => '1234'
                    },
          'keyC' => {
                      'keyB' => '5678'
                    }
        };

$VAR1 = {
          'keyB' => {
                      'keyC' => '5678',
                      'keyA' => '1234'
                    }
        };