you want this

  DB<150> sprintf "%d-%d-%d-%d", ('1234567890123456' =~ /(....)/g)
 => "1234-5678-9012-3456"
[download]

or this

  DB<151> sprintf "%d%d%d%d-%d%d%d%d-%d%d%d%d-%d%d%d%d", split //, '12
+34567890123456'
 => "1234-5678-9012-3456"
[download]

But I'd rather prefer this

  DB<153> join "-", ('1234567890123456' =~ /(....)/g)
 => "1234-5678-9012-3456"
[download]

TIMTOWTDI ... depends if you always make sure that the input is 16 characters long.

update

kennethk++ msged me that leading 0 are a problem in the first approach

  DB<160> sprintf "%d-%d-%d-%d", ('0123456789012345' =~ /(....)/g)
 => "123-4567-8901-2345"
[download]

this would fix it

  DB<161> sprintf "%04d-%04d-%04d-%04d", ('0123456789012345' =~ /(....
+)/g)
 => "0123-4567-8901-2345"
[download]

but honestly, I doubt I would stick with this approach.