You can also read and write to the same file using the IO::InSitu module. An example form the doco seems to suit your requirements (the in and out filenames can be the same)

use IO::InSitu;

my ($in, $out) = open_rw($infile_name, $outfile_name);

for my $line (<$in>) {
    $line =~ s/foo/bar/g;
    print {$out} $line;
}
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