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open FILE, "<whatever.txt" or die "Oops $!\n";
my @content = <FILE>;
...
open FILE, ">whatever.txt" or die "Oops $!\n";
print FILE @content;
close FILE;

my $last = '';
open FILE, "<whatever.txt" or die "Oops $!\n";
...
close TMP;
rename "tmp.txt", "whatever.txt";
unlink "tmp.txt";