After trying out your code, I get the output which I'm unable to interpret

It looks something like this

#!/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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        "You're only given one little spark of madness. You mustn't lose it."         - Robin Williams

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.

Aaron B.
Available for small or large Perl jobs and *nix system administration; see my home node.