open my $in, '<', 'infile.txt' or die $!;
open my $out, '>', 'outfile.txt' or die $!;
while( my $line = <$in> ) {
  $line =~ s/example/replacement/;
  print $out $line;
}
close $in;
close $out or die $!;
rename 'outfile.txt', 'infile.txt' or die $!;