I assume, that you want to strip off a fixed size header first and then process a list of records having another fixed size. You can set the input record separator ($/) to a scalar reference holding the record size (see perlvar). Might be slower than (sys)read, though.

#!/usr/bin/perl
use strict;
use Data::Dumper;

my ($n, $m) = (9, 3); # var-names as requested 
my @result;

$/ = \$n;             # record size := 9 characters
$result[0] = <DATA>;  # read one record (size: 9)

$/ = \$m;             # record size := 3 charcters
while (<DATA>) {
        # do something with the next block of 3 chars
        last if length != $m;  # stop at first non m-sized record
        push @result, $_;
}

print Dumper \@result;

__END__
123456789111222333444555666777888999xxxyyyzzzZ
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This prints:

$VAR1 = [
          '123456789',
          '111',
...
          'zzz'
        ];
[download]