This won't work because you are opening the filehandle only for writing. In fact it will clobber the contents of the file.

It should be:

# open FILE, ">filename.txt" or die $!; #output only, and deprecated s
+yntax

open my $file, '<', 'filename.txt' or die $!;
while (my $ethernet = <$file>) { 

  ## do something

}
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