The parentheses around split forces it to return an array. The square brackets are used to select a single element from the array returned by split. We could either use a constant numeric value, such as 3, or we could use a scalar variable, such as $col to select the array item. Keep in mind that Perl arrays start at 0, not 1. So, "one" becomes array element [0], and "four" is element [3]:

use strict;
use warnings;

my $col = 3;
my $str = "one two three four five";
my $output = (split /\s+/, $str)[$col];
print "output=$output\n";

__END__
output=four
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