That’s a good way to bypass blank lines, but it won’t bypass a line containing only spaces, or a line containing “word” characters and/or digits but no colons. In both these cases, there is nothing to split on, so $field[1] is undefined. Rather than trying to test the line to predict whether $field[1] will be defined, a simpler and safer approach is to test for the condition causing the warnings directly:

next unless defined $field[1];
[download]

Hope that helps,