The DATA filehandle is special; it reads from whatever comes after a __DATA__ line in your program. If you want to read from the file specified by $filename instead, you have to open it specifically.

open my $filehandle, '<', $filename
    or die "Can't read '$filename': $!";
while ( <$filehandle> ) {
    # etc.
}
close $filehandle or die "Fail on close '$filename': $!";
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