Excellent catch, and a great test case. My approach was wrong. In order to sort this in a DWIM-like way, this would probably work better:

my @old = ("10.5 AA", "100 NO");

my @new = map { join(' ',@$_) } sort { 
    ($b->[0] <=> $a->[0]) || ($b->[1] cmp $a->[1]) 
} map { ($1,$2) if m/(.*?)\s(.*)/ } @old;
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That's a bit cryptic. Basically, split each component of @old into number and string "columns", then use a two-criteria sort (sort on number, then string) and rejoin the "columns" into a string again.