#!usr/bin/perl
use strict;
use warnings;

my $nHeaderLines = 2;
my $nDataLinesPerFile = 4;

my @header;

# the "Big File" is the DATA segment below,
# maybe millions of lines...

for (1..$nHeaderLines) # read header lines from big file
{
   my $header_line = <DATA>;
   push @header, $header_line;
}

# divide the big file data into smaller files,
# each with the initial header...

my $nFile = 0;
my $fileNameBase = "SmallerFile";
my $nDataLine = 99999;

my $line;
while ($nDataLine++, defined ($line = <DATA>))
{
   if ($nDataLine > $nDataLinesPerFile) # start new file
   {
       $nFile++;
       my $name = "$fileNameBase$nFile.txt";
       open (OUT, '>', "./$name") or die "$!";
       print OUT @header;
       $nDataLine=1;
   }
   print OUT $line;
}

print "Program Done!\n";

__DATA__
Header 1
Header 2
data 1
data 2
data 3
data 4
data 5
data 6
data 7
data 8
data 9