...almost all image toolkits have advanced scale functions.... the best is the gimp...as it will give you the all the various options available in a scale.... but the perl automation to it, Perl-Gimp can be hard to get right..... ImageMagick functions ( see PerlMagick for all the options).....to be honest the ImageMagick maillist might be a good place to ask..... thay may have a 1 liner shell script

....if you really need to do it manually, it sounds like you want to read your file in 8 bits at at time, multiply it, then write it out to a separate file....sysread would be easy ... pack and unpack would be good too.... but a but more complex

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

my $file = shift || $0;
my $len =  8;

open FILE, $file or die "$!";

# write to your scaled file instead of stdout
while (sysread FILE, my $data, $len){ 
    # multiply your data here  
    syswrite STDOUT, $data };
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