Did you try your solution? It doesn't add a trailing newline (nor fix the more important problem). Here's my next stab: 

    s{(
        (?:[^\n]{1,15})(?=\n|$) # Line requiring no wrapping
      | (?:[^\n]{0,14},)(?!\n)  # Line that can be wrapped at comma
      | (?:[^\n]{15})(?!\n)     # Line to be wrapped, not at comma
      )\n?
    }{$1\n}gx;

[download]

And just two test cases: 

#!/usr/bin/perl -w
use strict;
my @lines= (
    join( '',
        "0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,",
        'c'x50,
        ",16,17,18,19,20,21,22,23,24,25,26,27",
    ),
    join( '',
        "0,1,2,3,4,\n5,6,7,8,9,10,11,12,13,14,15,",
        'c'x50,
        ",16,17,18,19,20,21,22,23,24,25,26,27",
    ),
);

for my $line (  @lines  ) {
    print "\n";

    $_= $line;
    s/([^\n]{0,14},|[^\n]{15})(?!\n)/$1\n/g;
    print "tye 1\n($_)\n";

    $_= $line;
    s/([^\n]{0,14},|[^\n]{15})(?=[^\n])/$1\n/g;
    print "ikegami\n($_)\n";

    $_= $line;
    s/((?:[^\n]{0,15}$)|(?:[^\n]{0,14},)|(?:[^\n]{15})(?=[^\n]))/$1\n/
+g;
    print "braveghost\n($_)\n";

    $_= $line;
    s/((?:[^\n]{1,15}$)|(?:[^\n]{0,14},)|(?:[^\n]{15})(?=[^\n]))/$1\n/
+g;
    print "braveghost +1\n($_)\n";

    $_= $line;
    s{(
        (?:[^\n]{1,15})(?=\n|$) # Line requiring no wrapping
      | (?:[^\n]{0,14},)(?!\n)  # Line that can be wrapped at comma
      | (?:[^\n]{15})(?!\n)     # Line to be wrapped, not at comma
      )\n?
    }{$1\n}gx;
    print "tye 2\n($_)\n";
}

__END__

tye 1
(0,1,2,3,4,5,6,
7,8,9,10,11,12,
13,14,15,
ccccccccccccccc
ccccccccccccccc
ccccccccccccccc
ccccc,16,17,18,
19,20,21,22,23,
24,25,26,       # Oops, extra newline
27)             # Oops, no trailing newline
ikegami
(0,1,2,3,4,5,6,
7,8,9,10,11,12,
13,14,15,
ccccccccccccccc
ccccccccccccccc
ccccccccccccccc
ccccc,16,17,18,
19,20,21,22,23, # (Exact same problems as above)
24,25,26,       # Oops, extra newline
27)             # Oops, no trailing newline
braveghost
(0,1,2,3,4,5,6,
7,8,9,10,11,12,
13,14,15,
ccccccccccccccc
ccccccccccccccc
ccccccccccccccc
ccccc,16,17,18,
19,20,21,22,23,
24,25,26,27

)               # Oops, /two/ trailing newlines
braveghost +1
(0,1,2,3,4,5,6,
7,8,9,10,11,12,
13,14,15,
ccccccccccccccc
ccccccccccccccc
ccccccccccccccc
ccccc,16,17,18,
19,20,21,22,23,
24,25,26,27
)               # Changing 0 to 1 fixes that
tye 2
(0,1,2,3,4,5,6,
7,8,9,10,11,12,
13,14,15,
ccccccccccccccc
ccccccccccccccc
ccccccccccccccc
ccccc,16,17,18,
19,20,21,22,23,
24,25,26,27
)               # Also works

...
braveghost
(0,1,2,3,4,     # Oops, extra newline

5,6,7,8,9,10,
11,12,13,14,15,
ccccccccccccccc
ccccccccccccccc
ccccccccccccccc
ccccc,16,17,18,
19,20,21,22,23,
24,25,26,27

)
braveghost +1
(0,1,2,3,4,     # s/0/1/ doesn't fix this

5,6,7,8,9,10,
11,12,13,14,15,
ccccccccccccccc
ccccccccccccccc
ccccccccccccccc
ccccc,16,17,18,
19,20,21,22,23,
24,25,26,27
)
tye 2
(0,1,2,3,4,
5,6,7,8,9,10,
11,12,13,14,15,
ccccccccccccccc
ccccccccccccccc
ccccccccccccccc
ccccc,16,17,18,
19,20,21,22,23,
24,25,26,27
)               # Yay!

[download]

(update) Note there are three ways to write part of that: (?=\n|$) or (?![^\n]) or (?m:$). The last can be written as just $ by putting the m option on the end of the s/// construct. I leave the choice up to you.

- tye        

In reply to Re^4: Using pos() inside regexp (fixed) by tye
in thread Using pos() inside regexp by braveghost

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