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

my $input = 'input.txt';
my $output = 'output.txt';
my $constant = 15; #number you are multiplying by
my $minimum  = -0.0001; # Zeroize numbers smaller than this
my $NO_DATA      = -9999;

open my $in, "<", $input or die "Could not open $input for reading:$!"
+;
open my $out, ">>", $output or die "Could not open $output for append:
+$!";
while  (<$in>){
  next if $. < 7; # Skip first 6 lines
  chomp $_;
  my @values = split;
  for (@values){
     if($_ == $NO_DATA){
          # leave -9999 alone
     } elsif( $_ < $minimum ){
       $_ = 0; # or $NO_DATA;  # replace small negative with 0 or -999
+9
     } else {
       $_ *= $constant;  # multiply other numbers by constant
     }
  }

  print $out join ( " ",@values),"\n";
}
close $out;
close $in;
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