A very full implementation might be:

sub best_size {
   my $r = shift;
   ($r > (1024 * 1024)) ? return sprintf("%1.3f", $r/(1024*1024)) . 'M
+B' :
   ($r > 1024)          ? return sprintf("%1.3f", $r/1024)        . 'K
+B' :
   return $r . 'B';
}
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This will return a string with the "most appropriate" suffix. Modify to suit your purposes, such as different precisions, or changing to the next suffix before actually reaching 1KB, 1MB, etc.