my $base = '/perl/bin/thisdir';
my $count;

opendir(BASE, $base) or die "Cant open $base: $!\n";

for (grep( !/^\./, readdir(BASE) )) {
   open(IN, "$base/$_") or warn "Cant access $_: $!\n" and next;
   while (<IN>) {
      $count += s/old/new/gi;
   }
   close(IN);
}

closedir(BASE);

print "Total changes: $count\n";