As for the error message, let's look at:

$^W = 1;
$index = 0..2; # this is your problem - it's initially undef
@matching = qw(disk duck deck);
$result = $matching[$index]; # and here's your problem, $matching[unde
+f]!
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You've done the .. operator in a scalar context, making it the flip-flop range operator, but you've not read any input, so you're not at line 0 yet, so the value is undef (outside the range so far). This value will be true only when $. is between 0 and 2, inclusively.

-- Randal L. Schwartz, Perl hacker