Thank you, @YourMother.

1. "first fail is the file reading line by line will break the records into two passes of your while(<$INPUT_REF_FH>){} and then . does not match newlines in regular expressions by default. Add an s flag to your regex to match it."----However, I validate input to make sure the <BIB> tags are within a single line, which is the correct way to tag files I have to use.

2. "The use of the x is meaningless in your regex"----Yes, I agree; I carried it over from my another expression that required multiple lines and comments in the searches.

3. Thanks to you, I have started to use ".+?" instead of ".*?"