use strict;
use warnings;

my %fh;

foreach (1..1000) {
   my $num = int(rand() * 10000);
   my $first_char = substr($num, 0, 1);
   $fh{$first_char} 
      || open($fh{$first_char}, ">$first_char.out") or die $!;
   print {$fh{$first_char}} "$num\n";
}