use strict;
use warnings;

no warnings 'numeric';

my %suffix = (K => 1, M => 1e3, G => 1e6, T => 1e9);
# or alternative values:
# my %suffix = (K => 1, M => 1024, G => 1024 ** 2, T => 1024 ** 3);

while(<DATA>) {
  my($col1, $col2) = split;
  print "$col1 - $col2 = ";
  my $suff1 = substr($col1, -1, 1);
  my $suff2 = substr($col2, -1, 1);

  if($suffix{$suff1} < $suffix{$suff2}) {
    # Alter $col2 and $suff2
    $col2 *= $suffix{$suff2} / $suffix{$suff1};
    $suff2 = $suff1;
  }

  elsif($suffix{$suff1} > $suffix{$suff2}) {
    # Alter $col1 - no need to alter $suff1
    $col1 *= $suffix{$suff1} / $suffix{$suff2};
  }

  $col1 -= $col2;

  print "${col1}$suff2\n";
}


__DATA__
324K    324K
440K    533K
23T    224G
42G    42G
1.9T    709G
294K    294K
684K    684K
492K    492K
62M    64M
48K    41M
34M    433K
317K    812K