It's a subscript, like used with an array, or in this case, a list.

my $value = ('a', 'b', 'c', 'd')[1];
say $value;
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...prints b

Very convenient as a quick and dirty tool in situations like this:

my $keep = ( split /\s+/, $string )[2];
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...although as with most things in Perl, that's only one of many ways to grab the third space-delimited item in a string.