If the "else" branch in line 51 is taken, your variable $delay_ATO_CTO_min will be "" when you reach line 60 and you'll get the warning you mention. Please consider:

if (($delay_ATO_CTO_min ne "") && ($delay_ATO_CTO_min > 10)) {
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Also, the warning for line 48:

Use of uninitialized value in concatenation (.) or string at ./630978.
+pl line
        66, <INFILE> line 1 (#1)
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is another clue that many of your values aren't initialized correctly. Many lines in your input file don't have any "aircraft information", just a timestamp. You should skip those lines (I guess). I think you also "split" on ";" when you should split on whitespace. Consider this instead...

    my @Parts = split('\s+');
    next if (scalar @Parts != 6);
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Good luck!