If your file is sorted as shown, you just have to read through it one line @ a time and check for a change of the first field in the record...warning untested but something like the following

 
use strict;
use warnings;
my ($FH, $OUT,$past,$record,$count);
open ($FH,"<","$ARGV[0]");
$record="";
$count=0;
while ($line=<$FH>){
   my($key ,@record)=split(/\s+/, $line);
   if $past ne $key){
      $count++;
      if ($count == 100){
         open ($OUT, ">" $past);
         print $OUT $record;
         close $OUT;
         $record="";
         $count=0;
      }
      $past=$key;
   }
   $record.= $line;
}
open ($OUT, ">" $past);
print $OUT $record;
close $OUT;
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Update: Added counter