I see that switching the opereands is better but I don't catch what you mean with "equal sorting keys do not keep their relative positions after the sort operation"... probably I miss something. Could you explain deeply that point? Thanks.

Sure! Let me do it with an example:

my @data = qw(1a 1b 2a 2b);
my @s1 = reverse sort { $a <=> $b } @data;
my @s2 = sort { $b <=> $a } @data;
print "@s1\n@s2\n";
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generates:

2b 2a 1b 1a
2a 2b 1a 1b
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Note how in the second row, elements with the same sorting key (the numeric part) appear in the same order they had in @data while in the first row, corresponding to the reverse solution, they do not.