Perl knows what you want to do, because that is what you have asked it to do:

# You use push so you must want an array
push @{ $provisions{$person} }, $1; 

# You use hash of hash addressing so you must want a hash
$total_bytes{$source}{$destination} += $bytes;
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The programmer has made the decision based on the data structure thought to be most useful in each case.

You can make it as crazy as you like and Perl will obey...

perl -MData::Dumper -le'$t{x}{y}[2]{foo}[3]{bar}=1; print Dumper \%t'
$VAR1 = {
          'x' => {
                   'y' => [
                            undef,
                            undef,
                            {
                              'foo' => [
                                         undef,
                                         undef,
                                         undef,
                                         {
                                           'bar' => 1
                                         }
                                       ]
                            }
                          ]
                 }
        };
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Cheers,
R.