Don't lexicalize them with my $a or my $b if you want to be able to use them in the sort() comparison block or function.

##</code><code>##

$ perl -we '
   local $a = 1;
   @h = sort {$a <=> $b} keys %hash;
   print "@h\n";> %hash = map { $_ , undef } 0 .. 3;
   @h = sort {$a <=> $b} keys %hash;
   print "@h\n";
'
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