It works the same as

use strict;
use warnings;

my %hash = (    A => 'a',
                B => 'b',
                C => 'c',
                D => 'd',
                E => 'e',
                F => 'f' );

my @x = %hash;
print "@x\n";
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Basically, you're imposing list context on the hash, which gives you back the key-value pairs (in semi-random order). The @{[ ... ]} construct is a way of imposing list context within an interpolating string. *shrugs* Neat corner case. :-)