What's wrong with this?

Just that Perl often Does What You Mean, doesn't mean you can just go try syntaxes you already know from other languages. If you want to do something you haven't done in Perl before, you should read documentation, because code like this is very unlikely to work. I suggeset you read the following documents:

• perllol
• perlreftut
• perlref
• perlsyn

Good luck finding out how Perl works.

Your code doesn't work for several reasons.

• Arrays are sized automatically in Perl, so with $foo[4], you're already using the fifth element of @foo. With $foo[4][8], you're using the ninth element of the array that is referenced to in the fifth element of @foo. To assign to @foo, use @foo.
• Anonymous array references are made with square brackets, not curlies. Perl does not have multidimentional arrays natively, but it can have arrays that hold references to other arrays. When not naming them, they are anonymous.
• Elements need to be separated with commas.

Here's an example matrix in Perl:

@foo = (
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
);
print $foo[2][1]; # prints 8
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