I imagine you have seen and copied syntax like:

my $data = do { local $/, <DATA> };
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... which Monks use a lot around here to demonstrate solutions using data contained in the script.

But in your case, if you are interested in handling the lines one at a time, and they exist in a separate file, there's no good reason to combine them all in a scalar.

open my $fh, '<', $filename or die $!;
while ( <$fh> ) {
    ... # process one line at a time
}
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Hope this helps!