G'day Dazz,

"Readability and maintainability are more important to me that speed or brevity."

That's great. Here's some suggestions:

• Avoid experimental features: they are subject to change. Some weeks ago I wrote "Re: Modern exposition of "case" or Switch in perl 5.34?". There is discussion about switch with several examples of alternatives.
• I question the use of Tie::File. Is there a specific reason you chose that? I would have thought a simple open would have been clearer: it's more readable and commonly understood (I suspect many would need to consult the documentation to understand the Tie::File syntax).
• Consider the autodie pragma. Hand-crafting I/O exception messages is tedious and error-prone. Write less code and let Perl do the work for you.
• On comments in your code:
  • In general, only use comments to clarify; not to reiterate or state the obvious.
  • Many are completely superfluous; for instance, "# Declare constants" immediately before "use constant ..." statements.
  • Some may become incorrect as the code matures; for instance, you have "mb_ip.cfg" in at least two comments, if those comments were necessary, which I question, use "$ip_filename" (if the filename changes, you won't have to search for and edit every occurrence of the hard-coded filename).
  • Some are just wrong to start with; for example, "# A hash ... in order ..." — hashes are unordered.
• On constants:
  • It is customary to use uppercase: e.g. TRUE and FALSE; not true and false. Keeping to expected conventions will improve readability.
  • Group your constants into one or more blocks; for example,

    use constant {
        TRUE  => 1,
        FALSE => 0,
    };
    [download]

  • Note that my ($x, $y, $z); declares three variables all of which will evaluate to FALSE; there is no need to individually assign FALSE to each.
• A final else (or default or whatever you use) should not contain a condition; that would be elsif.
• When specifying whitespace in a regex, do so explicitly. Instead of "/^.*static\s*ip_address=/", it would better to write /^\s+static\sip_address=/; that matches leading whitespace (tabs and spaces) and won't match "whateverstaticip_address=":

  $ perl -E 'my $x = "whateverstaticip_address=X"; say $x =~ /^.*static\
  +s*ip_address=/ ? "YES" : "NO"'
  YES
  
  $ perl -E 'my $x = "whateverstaticip_address=X"; say $x =~ /^\s+static
  +\sip_address=/ ? "YES" : "NO"'
  NO
  [download]

• Use consistent indentation. That will greatly improve readability and facilitate easier and less error-prone maintenance.

There's quite a few points. There may be others that I didn't pick up on my first pass through your code.