(Please use <code> tags to display code, not <pre>tags)

Do you have "use warnings" in your code? I ask because the important piece of your code seems to be if($fieldName eq $ourstring) and according to the extract you've shown us, $fieldName is undefined at that point.

But assuming that it's set somewhere outside of this extract, I still think that you can simplify a lot of this code. I'd consider setting up a hash where the keys are the column names and the associated values are the column numbers. Something like:

my %columns;
@columns{@split_first_line} = 0 .. $#split_first_line;
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You can then get the column number associated with a given field name with a simple:

if (defined (my $field_num = $columns{$field_name})) {
  # field exists
}
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Also, the best place to deal with chomping the input data is probably as soon as you've read it in, when a simple chomp(@f) would suffice.