Well, just break it apart:

[ ... ]
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creates an arrayref pointing to its contents, in this case FOO (not quoted, so we get the constant value). And then:

@{ ... }
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dereferences the enclosed value as if it were an array reference. So essentially, this is the hard way to say:

( ... )
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between the two of them, except that it also interpolates within a double-quoted string. That's how I stumbled across this in the first place: how to evaluate a value inside a double-quoted string. I wrote about it in a CLP posting early in the Perl5 days, and the meme stuck and is still being used in other places.