using a regex:

use warnings;
use strict;

my $string = "TE=ASR,EV=R,PPF=G1,TG=G1,TMT=P1,RTV=YE,AAT=0,AT=105,ON=Y
+,CF1=FGOC";
my $width = int (length ($string) * 0.28);
my @parts = $string =~ m/(.{1,$width},)(.{1,$width},)(.{1,$width},)(.*
+)/;

print join "\n", @parts;
[download]

Prints:

TE=ASR,EV=R,PPF=G1,
TG=G1,TMT=P1,
RTV=YE,AAT=0,
AT=105,ON=Y,CF1=FGOC
[download]