Simply open the files and use them (its best to open the input file first: if there's an error opening it, you don't want to create the output file).

  sub convert { scalar reverse shift } # whatever

  my $infile = "infile.txt";
  my $outfile = "outfile.txt";

  open IN, "<$infile" or die "can't read $infile: $!";
  open OUT, ">$outfile" or die "can't write $outfile: $!";

  while (<IN>)
  {
    print OUT convert($_);
  }

  close OUT;
  close IN;
[download]

If you're doing only simple conversions, then you don't need to put the formating in a subroutine; but it doesn't do any harm. --Dave.