To further clarify, the

while (<FH>) {
...
}
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construct is magical. Perl sees that, and does something like this:

while (defined ($_ = <FH>)) {
...
}
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Thus, use

while (($_ = <STDIN>) ne "\n"){
...
}
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to properly emulate this behavior. Also, I believe that use warnings would have detected improper use of numeric comparison, mate.