Can you give a scenario where do you use 1st case?

If I write a subroutine that receives a hash reference as an argument, I wouldn't modify it in-place, because the caller might want to re-use it.

Simplified example:

# caller:

my %h = (a => 1, b => 2);
something(\%h);
say $_ for keys %h;

# callee
sub something {
    my $arg = shift;
    $arg = {
       %arg,
       c => 3,
       d => 4,
    };
    # do more stuff with $arg here
}
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If you use $arg->{c} = 3 in sub something, %h changes in the caller's code, which is very likely to cause confusion.