$
$ cat a.txt
This is the first line in file a.txt
$
$ cat b.txt
This is the first line in file b.txt
This is the second line in file b.txt
$
$ cat c.txt
This is the first line in file c.txt
$
$

$
$ cat process_files.pl
#!perl -w
# Script to process all *.txt files in current directory and put the output
# in the corresponding *.txt.new files in the current directory again.
my $indir  = ".";  # the input directory name
my $outdir = ".";  # the output directory name
my $i = 0;         # loop index
my $infile;        # the input file name
my $outfile;       # the output file name
while (defined($infile = glob($indir."\\*.txt"))) {             # loop through the txt files in directory $indir
  printf("(%d)\tNow processing file => %s\t",++$i,$infile);
  $outfile = $outdir."\\".substr($infile,rindex($infile,"\\")+1).".new";  # name the output file
  open (IFILE, "<$infile") or die "Can't open $infile: $!";     # open input file for reading
  open (OFILE, ">$outfile") or die "Can't create file: $!";     # create output file for writing
  while (<IFILE>) {                                             # loop through contents of the input file
    # now do all your processing here
    # print to the output file
    print OFILE "We've processed this line -> ".$_;
  }
  close(IFILE) or die "Can't close $infile: $!";                # close input file
  close(OFILE) or die "Can't close $outfile: $!";               # close output file
  printf("Output file => %s\n",$outfile);
}
$
$

$
$
$ perl process_files.pl
(1)     Now processing file => .\a.txt  Output file => .\a.txt.new
(2)     Now processing file => .\b.txt  Output file => .\b.txt.new
(3)     Now processing file => .\c.txt  Output file => .\c.txt.new
$
$ cat a.txt.new
We've processed this line -> This is the first line in file a.txt
$
$ cat b.txt.new
We've processed this line -> This is the first line in file b.txt
We've processed this line -> This is the second line in file b.txt
$
$ cat c.txt.new
We've processed this line -> This is the first line in file c.txt
$
$