Since others have pointed out that the concatenation operator evaluates its operands in scalar context (which is what you should expect), just take it one step further and remember what happens when you evaluate an @array in scalar context? ....you get a number signifying the number of elements in the array. For example:

my @array = qw/a b c d e/;
print scalar @array, "\n";
__OUTPUT__
5
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Now, this creates two annonymous arrays referred to by the two first elements of @aoa;

my @aoa = ( [a, b, c], [d, e, f] );
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And when you dereference it like this, you evaluate one complete array:

my @array = @{$aoa[0]};
print scalar @array, "\n";
__OUTPUT__
3
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Well, your code snippet simply evaluates in scalar context two individual anonymous arrays, thus concatenating together the value representing the number of elements in each of those two anonymous arrays held in @aoa.

The result of '5' . '5' is '55'.... nothing surprising if you walk through what's happening.