Records of a particular length may be denoted by starting with particular characters or by the length of the record.

Some of the brethren have boggled at fixed-width files that mix different record lengths, but I think we can make some sense of this, especially if the record type is signaled by the initial character.

use strict;
use warnings;

my %histogram;
my %records_of_length;
while (<DATA>) {
    my $record_length = length;
    my $initial_char  = substr($_, 0, 1);
    $records_of_length{$record_length}++;
    $histogram{$record_length}{$initial_char}++;
}

# Review how many distinct record lengths were seen.
# If all records of given length start with same char,
# rejoice!
for my $rec_len (sort {$a <=> $b} keys %histogram) {
    print "Saw $records_of_length{$rec_len} records";
    print " with length $rec_len:\n";
    for my $char (sort keys %{$histogram{$rec_len}}) {
        print "\t$char: ";
        print $histogram{$rec_len}{$char}, "\n";
    }
}
__DATA__
C4498 John__ Smith___
I0023 widget 004   4.95
I0869 foozle 001  29.50
I7765 gadget 002 340.00
C5678 Mary__  Doe____
I9999 misc__ 003   6.25
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prints

Saw 2 records with length 22:
        C: 2
Saw 4 records with length 24:
        I: 4
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and now you can work on heuristics to decide if the number of different record types is small enough to usefully classify the file as "mixed fixed width".