In addition to relying on the return of keys or values, a hash is just a list. It can be seen an array where there exists an ordering between each tuple (key, value). BUT there is no ordering among tuples. The => is also known as a fat comma.

This is why you can assign a hash, equivalently like:

my %hash1 = ('keyA' => 'valA', 'keyB' => 'valB');
my %hash2 = ('keyA', 'valA', 'keyB', 'valB');
my %hash3 = ('keyA' => 'valA'=> 'keyB' => 'valB');
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And use it to capture arguments in a subroutine into a hash, like:

sub mySub {
  my %args = @_
  #... do stuff
}
# and call it equivalently like
mySub ('keyA' => 'valA', 'keyB' => 'valB');
mySub ('keyA', 'valA', 'keyB', 'valB');
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This is also why you may see the error, "Odd number of elements in anonymous hash". It's because list assignment to a hash must have an even number of elements (i.e., key/value pairs).

Finally, to get back to your question. It is not only safe to use join on the output of keys, but also on the hash itself:

my $stringA = join ',', %hashA;

#additionally, this is valid (to further demonstrate the point):
my @arrayA = %hashA;
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It bears repeating that hash to array assignment, key/value ordering between pairs is necessarily retained, but ordering among tuples is not guaranteed.

update: fixed spelling of comma!