Alternately, although this is less readable, you can dereference a hash in the subroutine with:

foo(\%hash);

sub foo {
    my %hash = %{shift()};
    # rest of function here, which manipulates hash directly
}
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Call me perverse, but I just love the way that line looks (from what I understant, the () is required when you put shift in the braces to force perl to recognize it as a function call and not a hash key ... but I've been wrong about these things in the past ...