I suspect I don't understand the question (or Perl) but...

If you are trying to insert an arbitary string wouldn't s/// do the trick?

use strict;
use warnings;
my $f="Joe Bloggs, 1615 Thislane Street, MyTown, NZ, 1851\n";

my $in=" This is a new longer address";

print $f;
$f=~s/,.*/,$in/s;
print $f;
[download]

--mandog