The syntax isn't specific to the TK::* tags stuff, it's a general syntax for array references.

There are basically two ways to create such a beast. On is to use an array, and pass a reference to it:

my @array = (1, 2, 3);
my $array_ref = \@array; # the \ takes the reference
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The other way is to use square brackets to create an anonymous array reference:

my $array_ref = [1, 2, 3];
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More on this, and the difference between the two, can be found in perlreftut.