How about a find/rewind approach ? Go ahead and set $/ to your delimiter, but don't assign to a variable as you read. (for the sake of conserving memory on huge records)

Once you have the record boundaries, then you could seek() back and process it however you need to.

This is making the assumption that the bare <FILE> construct isn't using memory. I'm not clear on whether that's true or not.

#!/usr/bin/perl
my $delim="asdf";
$/=$delim;
open(FILE,"/some/big/file");
my $offset=0;
my $lastoffset=0;
for (;;) {
   <FILE>;
   my $offset=tell(FILE);
   last if eof(FILE); 
   print "this record spans [$lastoffset] to [$offset], including";
   print "the delimiter\n";
   print " this means you could seek to $lastoffset and read for ";
   printf "%d bytes to get the record\n",
       $offset-$lastoffset-length($delim);
   $lastoffset=$offset;
}
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