#!/usr/bin/perl

main:
{

    $argc = @ARGV;
    if ($argc < 2) {
        print "usage: ./sub old_string new_string file1 [file2 file3 ...]\n";
        exit(0);
    }
    $old_string = shift(@ARGV);
    $new_string = shift(@ARGV);
 
    foreach $file (@ARGV){
        update($file, $old_string, $new_string);
    }
 
    exit (0);

}

sub update {
  local ($filename) = $_[0];
  local ($old_string) = $_[1];
  local ($new_string) = $_[2];
  local (@lines, $file);
   
  open(FILE, $filename);
  @lines = <FILE>;
  $" = ""; # no spaces between elements when converting array to scalar
  $file = "@lines";
  close(FILE);
 
  $file =~ s/$old_string/$new_string/gsi;
 
  open(FILE, ">$filename");
  print FILE $file;
  close(FILE);
}