($var =~/(?<=use )neighbor[[:alpha:]-]* ([[:alnum:]\.-:]+)/) and do{
    print $1;
};

##</code><code>##

Compiling REx `(?<!use )neighbor[[:alpha:]-]* ([[:alnum:]\.-:]+)'
size 42 Got 340 bytes for offset annotations.
first at 1
   1: UNLESSM[-4](7)
   3:   EXACT <use >(5)
   5:   SUCCEED(0)
   6:   TAIL(7)
   7: EXACT <neighbor>(10)
  10: STAR(23)
  11:   ANYOF[\-A-Za-z+utf8::IsAlpha](0)
  23: EXACT < >(25)
  25: OPEN1(27)
  27:   PLUS(40)
  28:     ANYOF[.-:A-Za-z+utf8::IsAlnum](0)
  40: CLOSE1(42)
  42: END(0)
anchored `neighbor' at 0 floating ` ' at 8..2147483647 (checking anchored) minlen 10 
Offsets: [42]
	8[4] 0[0] 5[4] 0[0] 8[0] 8[0] 10[8] 0[0] 0[0] 30[12] 18[12] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 31[1] 0[0] 32[1] 0[0] 48[15] 33[15] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 49[1] 0[0] 50[0] 
Guessing start of match, REx `(?<!use )neighbor[[:alpha:]-]* ([[:alnum:]\.-:]+)' against `use neighbor 2001:504:0:4::6181:1'...
Found anchored substr `neighbor' at offset 4...
Found floating substr ` ' at offset 12...
Starting position does not contradict /^/m...
Guessed: match at offset 4
Matching REx `(?<!use )neighbor[[:alpha:]-]* ([[:alnum:]\.-:]+)' against `neighbor 2001:504:0:4::6181:1'
  Setting an EVAL scope, savestack=5
   4 <use > <neighbor>    |  1:  UNLESSM[-4]
   0 <> <use neighbor>    |  3:    EXACT <use >
   4 <use > <neighbor>    |  5:    SUCCEED
                              could match...
                            failed...
Match failed
Freeing REx: `"(?<!use )neighbor[[:alpha:]-]* ([[:alnum:]\\.-:]+)"'