Since you are new to coding my guess is because @columnsA (or whatever) has only 2 elements while your code thinks there are three. Arrays are indexed from zero upwards, not from one.

This is a good suggestion, but there should be 7 elements in the array (or 0-6).

What steps have you undertaken to ascertain your assumption?

For example, the following prints the number of elements in the array @columns:

print 0+@columns;
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And the following shows you the values of the elements:

use Data::Dumper;
print Dumper \@columns;
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Personally, I would always look at the line where @columns gets initialized, and also inspect the source data, by printing the line $line1.