You could also open the file for reading, read it into an array, close it, filter the values, then open the file for writing (which will create a new file so those old lines won't be hanging around) and write to it.

open my $fh, "data" or die "could not open data for reading: $!";
my @data = <$fh>;
close $fh;

open $fh, ">data" or die "could not open data for writing: $!";
for (0..4) {
        print $fh $data[$_];
}
close $fh;
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