Though bless indeed requires a reference as argument it actually tells the thingy referenced by REF that it is now an object, so it blesses the thingy, and not the reference:

use 5.010;
use strict;
use warnings;

my %hash;

say ref \%hash;
bless \%hash, "Whatever";
say ref \%hash;
__END__
HASH
Whatever
[download]

PS of course you can bless reference too:

use 5.010;
use strict;
use warnings;

my %hash;
my $ref = \%hash;

bless $ref, 'Array';
bless \$ref,  'Whatever';
say ref \%hash;
say ref \$ref;
__END__
Array
Whatever
[download]