There are a few problems. One is that %s{'a'} = 10; won't compile. You mean to say, $s{'a'} = 10;

The next problem is that testa() returns a flat list of keys and values, not a hash. You could re-hashify them like this:

%{ { testa() } }
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This takes the return value of testa() (a flat list of keys interlaced with values), absorbs that list into an anonymous hash (the inner set of {...} does this), and then dereferences that anonymous hash ref (the outter %{ ..... } does this).

But even then you still have a problem. Your loop will print a list of keys, sure enough. But how are you going to access the values if nothing is holding onto that hash ref?

Try this instead:

while( my( $key, $val ) = each( %{ { testa() } } ) {
    print "Key: $key\tValue: $val\n";
}
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...or better yet, pass by reference, as described in perlsub:

while( my( $key, $val ) = each( %{ testa() } ) {
    print "Key: $key\tValue: $val\n";
}
sub testa {
    my %s = (
        a => 10,
        b => 20,
        c => 30
    );
    return \%s;
}
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