Line 8 is " $pages{$page}++; ".

If your input line ($line) is blank/empty, $page would be undef.

This would result in The warning message you get.

The fix is to check that you do get data - above line 8, add the line:

    next unless $page;
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FYI - Line 17 refers to the input data line 17, which you have not shown in your post, probably because it was not visible - just a blank line.