localtime returns a list and  ( )[0..2] is a list slice that narrows down that list to the first three elements.   localtime also accepts a numerical argument (usually time) but the number you are supplying in the second example is a reference to an anonymous array (localtime[0..2] is the same as localtime\@array).  But as you are reading from the beginning of the list you don't really need the list slice at all:

my ($sec, $min, $hour) = localtime;
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Will do the same as the first example.