I think I also prefer the join/unpack approach, but here's a regex with the highly prized "one-liner" property that works at least back to 5.8.9:

c:\@Work\Perl>perl -wMstrict -le
"print qq{perl version $]};
 ;;
 for my $cc (qw(
   1234987656785432 0000000000000000 0001000200030004 1000200030004000
   )) {
   print qq{'$cc'};
   ;;
   (my $fcc = $cc) =~ s{ \B (?= (?: ....)+ \z) }{-}xmsg;
   print qq{'$fcc' \n};
   }
"
perl version 5.008009
'1234987656785432'
'1234-9876-5678-5432'

'0000000000000000'
'0000-0000-0000-0000'

'0001000200030004'
'0001-0002-0003-0004'

'1000200030004000'
'1000-2000-3000-4000'
[download]