my ($subdir, @keywords) = split;
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is the same as

my @array = split;
my ($subdir, @keywords) = @array;
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is the same as

my @array = split;
my $subdir   = $array[0];
my @keywords = @array[1..$#array];
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In other words, $subdir gets whatever's in front of the first space, and @keywords gets all the other fields returned by split (if any).