What would be the best way to convert this comma delimited file into a fixed length one?

I'm not 100% sure, but is this what you are after?

#!/usr/bin/perl -w
 
use strict;
 
while (my $line=<DATA>) {
  chomp $line;
  my @f=split(",",$line);
  my $fmt = sprintf("%s","%10s" x 5) . "\n";
  printf $fmt,@f;
}
 
__END__
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
123456,12/01/2004,ABCDEFG,12345 ,XYZ
[download]

When that code is run you get:

    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
    12345612/01/2004   ABCDEFG    12345        XYZ
[download]

Not sure what you mean by "fixed length" or what your criteria for the proper field width is, but the principle remains the same.

In future you'd get better responses if you are more specific about what your requirements are.