Since the fields never change length it would seem that unpack is a better choice than substr, is that correct?

Perhaps. Read perlpacktut, which compares the various approaches. Here is an example of using unpack to parse each line into an array:

use warnings;
use strict;

while (<DATA>) {
    chomp;
    my @cols  = unpack 'A1A15A14A16', $_;
    print join(',', @cols), "\n";
}

__DATA__
C4432882490H019000020150211ESL6690 0H2015PC
C4833076550HC0P0000201412093J46651 0H2015DX
C6033106980H057130020150323FRE7602 0H2015PC
C663160140MT007015G20141124274847A MT2015PC
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