ancestry data filter

#!/usr/bin/perl

use 5.10.0;
use strict;
use warnings;
 
my $MAX_LINES = 100_000;
my @FIELDS = (7, 8, 9, 14, 17, 18, 19, 25, 30, 31, 35, 36, 37, 43, 44, 46, 47, 50, 52, 55, 62, 63, 66, 67, 68, 75, 76, 77, 81);

sub nextfilename {
  my $num = shift;
  return "data$num.txt";
}

sub getdatafields {
   my $line = shift @_;
   my @fields = @_;
   my @F = split /\|/, $line;
   return join "|", @F[@fields];
}

my $infilename = shift @ARGV or die "no input specified";
 
my $infile;
open $infile, $infilename or die "can't open data file";
my $outfile;
open($outfile, ">", nextfilename( 0 ))
or die "can't open " . nextfilename(0) . " for writing";

my $header_row = getdatafields(<$infile>, @FIELDS);
print $outfile $header_row;
 
my $i = 1;
for my $line (<$infile>) {
 
  if (($i++ % $MAX_LINES) == 0) {
    my $k = int($i/$MAX_LINES);
    my $l = $i - 1;
    print "working on file ". $k . "starting at observation # " . $l . "\n"; 
    close $outfile;
    open($outfile, ">", nextfilename( int($i/$MAX_LINES) )) or die "can't open " . nextfilename(int($i/$MAX_LINES)) . " for writing";
    my $headarrow = <$infile>;
    print $outfile $header_row;
  }
  say $outfile getdatafields($line, @FIELDS);
}
 
close $outfile;