Instead of looking at the next line, consider looking at the last line and the current line:

my $lastline;
while (my $currentline = <$fh>) {
    chomp $currentline;
    my @tmp = split /\s+/, $row;
    ...
    $lastline = $currentline;
}
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Alternatively, you can look at accessing the file as an array, using Tie::File.

If you're dealing with CSV data, consider using Text::CSV_XS.