but I can't understand what is going when I do %hash->{key}, kindly explain

There is nothing to understand or explain, other than that %hash->{key} should have triggered a syntax error in Perl. Some older Perls didn't and therefore this error was downgraded to a warning to avoid breaking existing scripts.

A reference should always be a scalar (which can be part of an array, a hash or indeed an object or the return value of a subroutine, but not the array, hash, ... itself.

BTW you will trigger a similar warning with the following:

@array = qw /1 2 3 4 5/;
print @array->[1];
[download]

It really is the same issue.

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