Improved by adding another S3 DEFAULT_HOST, putting the keys on the command-line.

rename-on-s3.rb

# This is a quick script for doing a mass rename of all files in an
# Amazon S3 bucket.
# In this case, the rename operation was to unescape all filenames which
# had been previously escaped in error.

#############################
# Configuration:

bucketname = ENV.fetch('S3_BUCKET_NAME')
access_key = ENV.fetch('S3_ACCESS_KEY_ID')
secret_key = ENV.fetch('S3_SECRET_ACCESS_KEY')

#############################

require 'rubygems'
require 'aws/s3'
AWS::S3::DEFAULT_HOST.replace 's3-eu-west-1.amazonaws.com' # You may have to change this.

include AWS::S3

require 'cgi'

# Some obscure regular expression you do not need.
RE = /\d{4}\-\d{1,2}\-\d{1,2}_\d{10}_\d+_google_[a-z\.]+_(\d+)\.tar\.gz/

def new_name(old_name)
  search_id = RE.match(old_name).captures.first
  "scrape-#{search_id}.tar.gz"
end

Base.establish_connection!(access_key_id: access_key,
                           secret_access_key: secret_key)

b = Bucket.find(bucketname)
marker = ''
while b.size > 0
  puts "\n\n--------------------new page----------------------"
  puts "\n From marker #{marker}"
  puts "\n\n--------------------------------------------------"
  b.each do |s3o|
    next unless RE =~ s3o.key
    begin
      S3Object.copy(s3o.key, new_name(s3o.key), bucketname)
      puts "Copied #{s3o.key} to #{new_name(s3o.key)}"
      # Uncomment this if you're feeling confident and want to delete the key
      # s3o.delete
    rescue => e
      puts "\n\n @@@@@@@@@@@@ EXCEPTION on key #{s3o.key} \n\n"
      puts e.message
      puts '@@@@@@@@@@@@@@}'
      next
    end
  end
  marker = b.objects.last.key
  b = Bucket.find(bucketname, marker: marker)
end