use strict;
use warnings;

use File::Find::Rule qw( );
use Path::Class      qw( file );

for (
   File::Find::Rule->name( '*.bmp' )
                   ->file()
                   ->in( $dir );
) {
   my $file = file($_);

   my $o = $file;
   my $n = $file->dir()->file(
       'reference_' . $file->basename()
   );

   if (-e $n) {
      warn("$n already exists\n");
      next;
   }

   if (!rename($o, $n)) {
      warn("Unable to rename $o: $!\n");
      next;
   }
}