# Set your input and output file
my $inputfile = "input.txt";
my $outputfile = "output.txt";

# Open your input file
open(my $input, $inputfile);

# Open your output file, for writing
open(my $output, ">$outputfile");

# Read each line in, one at a time
while(<$input>) {

     # Here's where you put your personal logic

     # When you need to print to the output file, do this
     # The $_ means, in this case only, "current line"
     print $output "Hey, I found $_\n";

}

# Close your files when done!
close($input);
close($output);