#!/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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The code I showed was not complete; it was just portions to demonstrate how to do the number changes. It's customary to show your code that you need help with, so we can point out what's causing the errors.