#!/usr/bin/perl
use POSIX;

my @outfile = ('file1.txt', 'file2.txt', 'file3.txt' , 'file4.txt');
my $infile = 'infile.txt';
my $lineno = 0;

open (my $ifh, '<', $infile);
while (<$ifh>) {
        my $out = $outfile[floor($lineno / 5)];
        open (my $fh, '>>', $outfile[floor($lineno / 5)%4]) or die "Unable to open outfile";
        print $fh $_;
        close $fh;
        $lineno++;
}
close ($ifh);