my $filename = 'foo.txt';

open IN, "< $filename" or die "Can't open $filename: $!\n";
open OUT, "> $filename.bak" or die "Can't write to $filename: $!\n";
while (<IN>) {
  s/foo/blah/g;
  print OUT $_;
}
close IN; close OUT;
rename "$filename.bak", $filename;