When you test the syntax of a perl expression, start off with simple cases:

$ perl -wE 'say keys [1, 2]'
01
$ perl -wE 'say keys { a => 1, b => 2 }'
ab
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So, works fine. Something seems to be off in your code, reducing it to a few essential lines probably shows the problem more clearly.

It also generally helps if you're more specific than "doesn't seem to work" -- do you get an error message, or some wrong result? What do you expect, what do you get?

Update: I removed some irrelevant parts from your code, and got an empty line, which makes sense since you haven't posted any code that initializes %chassis. When I add some initialization, it works as expected:

$returns{'do_stuff'} = do_stuff();

print keys($returns{'do_stuff'}->[2] ) , "\n";


sub do_stuff {
    my %chassis = ( a => 1, b => 2 );
    my @returns = (0 , "Success" , \%chassis);
    return(\@returns);
}
__END__
ab
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(Tested with perl 5.14.1).