Something like this may work (UNTESTED):

#!/usr/bin/perl
use warnings;
use strict;

$inFile = shift or die "usage: $0 filename\n";
open DATA, '<', $inFile or die "Cannot open '$inFile' $!";

my ( $file_count, %records );
while ( my $line = <DATA> ) {
    if ( 1 == $. || 100 == keys %records ) {
        ++$file_count;
        open OUT, '>', "file$file_count" or die "Cannot open 'file$fil
+e_count' $!";
        %records = ();
        }
    my $key = substr $line, 0, 9;
    $records{ $key }++;
    print OUT;
    }
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