#!/usr/bin/perl -w

use strict;

my @array = glob("*.htm");

for my $i(@array) {
  open(FH, "$i");
  open(NH, ">tmp");
    while(<FH>) { 
      $_ =~ s/string1/string2/g;
      print NH $_;
    }
  close(NH);
  close(FH);
  rename("tmp", $i);
}