I do not understand your overall question.

However, some specific comments on the OPed regexes:

• Inside and at the very beginning of a character class, the  ^ regex metacharacter produces class "inversion", i.e., the class matches any character not in the class. So  [^$find1] is the same as  [^%] and matches any character that is not a  '%' character.
• Outside a character class, the  ^ metacharacter asserts the beginning of the string (if the  /m regex modifier is not used). So  [^$find1]?^ is the same as  [^%]?^ and matches zero or one of any character that is not a  '%' and that is before the beginning of the string. Effectively, it matches the beginning of the string and a  '^' character is always inserted at the beginning of the string by the substitution.
• The expression  [$find1]?$ matches zero or one  '%' characters at the end of the string (or before a newline at the end of the string). Effectively, a  '+' character is always inserted at the end of the string by the substitution.

Update:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $field2_b = 'abcd';
 ;;
 my $find1 = '%';
 if ($field2_b ne '') { s/[^$find1]?^/\^/, s/[$find1]?$/+/ for $field2
+_b };
 print qq{'$field2_b'};
"
'^abcd+'
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Please see perlre, perlrecharclass, perlrequick, and especially perlretut.