G'day ddrew78,

"Any wisdom at all would be greatly appreciated."

It would be wise to:

• ask Perl to do some basic checks for you (see strict and warnings);
• do some basic checking yourself (e.g. does @ip_fields contain exactly four elements all of which match /^\d+$/);
• use lexical variables (e.g. $tunip is not lexical); and,
• show us actual input and output, which might include warning or error messages, such that we can comment on why particular input "just leaves '$bgp_ip' blank" (see "How do I post a question effectively?").

Furthermore, your descriptive text and programmed logic are at odds with each other:

$ perl -E 'my @x = split /\./, "10.1.1.5"; $x[3] % 2 ? --$x[3] : ++$x[
+3]; say join ".", @x'
10.1.1.4
$ perl -E 'my @x = split /\./, "10.1.1.6"; $x[3] % 2 ? --$x[3] : ++$x[
+3]; say join ".", @x'
10.1.1.7
[download]

You could reverse the logic to match the text:

$ perl -E 'my @x = split /\./, "10.1.1.5"; $x[3] % 2 ? ++$x[3] : --$x[
+3]; say join ".", @x'
10.1.1.6
$ perl -E 'my @x = split /\./, "10.1.1.6"; $x[3] % 2 ? ++$x[3] : --$x[
+3]; say join ".", @x'
10.1.1.5
[download]

But then you'd have edge case problems not addressed by either your description or code:

$ perl -E 'my @x = split /\./, "10.1.1.0"; $x[3] % 2 ? ++$x[3] : --$x[
+3]; say join ".", @x'
10.1.1.-1
$ perl -E 'my @x = split /\./, "10.1.1.255"; $x[3] % 2 ? ++$x[3] : --$
+x[3]; say join ".", @x'
10.1.1.256
[download]

So, there's potentially issues at a fundamental level (e.g. problem analysis, solution design, coding logic); and there's clearly problems with a lack of basic checking and validation.