BrianP,

I tried to stay out of this one, but 'what the heck!'

The type of work your doing requires a deeper understanding of Perl than you want to spend time on. In your write-up you mention passing a hash from a subroutine back to the main program, but failed to include that in your limited code description. As others have already pointed out, passing a hashref to the subroutine would have avoided having to copy a 27MM key/value hash back to the main program. As you show below it looks like it's in the main program anyway.

Tiny, active code segment
...    
$sr_len = sysread(IN, $buf, $bsize);  # SysRead Length    
last  if $sr_len == 0;      
while($buf)  {       
    $rgb=substr($buf, 0, 6, '');  # Nibble 6 bytes       
    $rgb2c{$rgb}++;    
}
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But why didn't you build the array while building your hash???

    
@rgb = keys %rgb2c;  << 1 line takes 28.648 min
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The above code is probably not doing what you think. '@rgb' is not in any specific order. Here's where knowing how Perl allocates an array and a hash, you could have done the following ( untested code ):

my $fsize = -s [your file];   ## Find out how big the image is?
my $arrsize = $fsize / 6;     ## Size of the array and hash
my $counter = 0;
my %rgb2c;
keys %rgb2c = $arrsize;       ## Allocate one large memory hash!
my @rgb[$arrsize] = '';       ## Allocate one large memory array!
while ( 1 )
{   $sr_len = sysread(IN, $buf, $bsize);  # SysRead Length    
    last  if $sr_len == 0;      
    while($buf)
    {   $rgb=substr($buf, 0, 6, '');  # Nibble 6 bytes       
        $rgb2c{$rgb}++;
        $rgb[$counter} = $rgb;        # Build array as you go along
        $counter++;
    }
}
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At this point you have a hash for telling you the number of colors and an array that represents the exact image in 48 bit increments. By pre-allocating the hash and array you make only one call to the operating system for memory for each, instead of millions of calls.

Spend a little more time learning Perl and using efficient algorithms, and you'll have tools that will make you proud.

Regards...Ed