I would just add another test:

use strict;
use warnings;

my $var = "use neighbor 2001:504:0:4::6181:1";
($var !~ /use neighbor/) and ($var =~/neighbor[[:alpha:]-]* ([[:alnum:
+]\.-:]+)/) and do{
    print $1;
};
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especially as I am not as literate with (negative)? look(back|ahead|around) assertions.