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#!usr/bin/perl
my $a = 1;
%hash = map { $_ , undef } 0 .. 3;    
...

__END__
0 3 2 1

@h = sort {$a <=> $b} @arr = keys %hash; # doesn't sort
@h = sort {$a <=> $b}        0 .. 3;     # do      sort
@h = sort {$a <=> $b} @arr = 0 .. 3;     # do      sort