You should have specified that in your original question. "... to be sure they only have new lines" is not the same as "give a understanable error to the user." Now then, here's how i might check for such an "error": 

my @array = <FILEHANDLE>;
if (@array < 1) {
    # we have no lines, send an error message
}
eslif (@array > 1) {
    # we have a 'too many lines' error jim, send a message
}
else {
    # this porridge tastes *just* right ... continue with processing
}

[download]
When you slurp a filehandle into an array, you can always count the number of lines by counting how many elements the array has. No need to explicitly check for newlines.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In reply to Re^3: How to test for a new line by jeffa
in thread How to test for a new line by BadMojo

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