#Fill in ...'s with more code as needed.
use strict;
use warnings;

my $filename = 'somelog.log';
my $outfilename = 'mungedlog.log';

my $oldHeader = '     id pool                 type rid rset                  min  max size used load';

# Define the output format, and where the data comes from
my @newFormatColumns = (
{title=>'date', length=>10, format='s', splitDataIndex=>15},
...
{title=>'id', length=>3, format=>'d', splitDataIndex=>0},
...
);

# Build new header lines automagically based on above AoH
my $newHeaderFormat;
$newHeaderFormat .= '%'. $_->{length} . 's' for @newFormatColumns;
my $newHeader = sprintf($newHeaderFormat, ( map {$_->{title}} @newFormatColumns));

open my $iFH, '<', $filename or die "Can't open '$filename' because: $!\n";
open my $oFH, '>', $outfilename or die "Can't open '$outfilename' because: $!\n";

#Grind through file until done.
while (my $line = <$fh>)
{
  chomp $line;
  if ($line =~ /.../) # looks like data
  {
    # Munge data lines into new format
    # grab data from the line read
    my @data = split /\s/, $line;
    # add new values to the end
    push @data, 'new values here', 'here', 'and here';
    
    # Build new line of data
    $line = '';

    # this is basically a set of:
    # $line .= sprintf ("%20s", $data[1]);
    # where $data[1] is the pool string for example.
    # and the '20', the 's' and the '1' all come 
    # from the AoH table declared at the beginning.

    $line .= sprintf ("%$_->{length}$_->{format}", $data[$_->{splitDataindex}]) for @newFormatColumns;

  }elsif ($line eq $oldHeader){
    # Change header lines
    $line = $newHeader;
  }else{
    # Leave unrecognized lines alone.
  }
  # And don't forget to print it all to the output file.
  print $oFH "$line\n";
}