use warnings;
use strict;

my %hash= ('a' => {'Size' => 40 },
	   'b' => {'Size' => 3000 },
	   'c' => {'Size' => 20 },
	   'd' => {'Size' => 1 },
	   'e' => {'Size' => 12 } 
);


my @unsort= map ( $hash{$_}->{"Size"} .':' . $_ , keys %hash);

no warnings;
my @sort= sort {$a<=>$b } @unsort;
use warnings;

foreach (@sort)  { 
  s/[^:]*://;
}

print join'  ',@sort,"\n";

# prints

d  e  c  a  b