s/frog/toad/g txtfile 

##</code><code>##

#!/usr/bin/perl -w
use strict;

# this script will edit itself
ReplaceStuff($0,'word','taco');

sub ReplaceStuff{
  my ($file,$origword,$newword) = @_;
  local $^I="";        # set the backup tag to nothing
  local @ARGV = $file; # make a local @ARGV so we can use <>
  # go through each line and do the substitution
  while (<>) {
    s/$origword/$newword/g;
    print;             # print stuff out
  }
}