The problem is solved; thanks for your patience with my ill-formed question that turned out not to be the problem at all.

# split-unix.pl -- do split in perl
#
# usage:split [-num] file [outname]

use strict;

my $outfile = "x";  #default
my $limit = 1000;
my $readfile;
my $s1= "a";

if($ARGV[0] =~/^-?[0-9]+/){
  $limit = 0- $ARGV[0];
  shift @ARGV;
}
$readfile = $ARGV[0];
shift @ARGV;
if (defined $ARGV[0]){
  $outfile = $ARGV[0];
}
$outfile.=$s1;
  
open (OUT, ">", $outfile) or die "$0: can't open $outfile $!";  
open (IN, "<", $readfile) or die "$0: can't open $readfile $!";
print OUT $limit;

#=pod
while(<IN>){
  if ($. > $limit ){
    close OUT;
    $outfile++;
    open(OUT, ">", $outfile) or die "$0: can't open $outfile $!";
  }
  print OUT "$_";
} 

#=cut
close OUT;
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