Not really sure how to make it recurse subdirectories other than adding /* on the end for each level needed:
 # perl -p -i.bak -e 's/bilbo/frodo/g' */*

Then we are back at File::Find.

#!/usr/bin/perl

use File::Find;
my @dirs = @ARGV;
@ARGV = ();
File::Find::find(
  {
    wanted => sub { push @ARGV, $File::Find::name if -f }
  },
  @dirs
);
$^I = '.bak';
local $_;
while ( defined( $_ = <ARGV> ) ) {
  s/bilbo/frodo/g;
  print;
}
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