I had thought of this too, but the OP claimed that each of the keys and the values are unique. He didn't exclude that any of the keys may also be a value, although it is definitely reasonable to doubt about this. I'm pointing this out just to let him/her know that this solution may fail in the case it is so:

use Data::Dumper;

my %hash=(foo => 'bar', bar => 'baz');
%hash=(%hash, reverse %hash);
print Dumper \%hash;
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Incidentally I wouldn't have used your smart and elegant syntax -of which I may have thought, but but which didn't occur to me- and I would have done a clumsier

@hash{values %hash} = keys %hash;
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instead.