Another way is to use a Schwartzian Transform passing the key, an indicator whether the cpu is "-" (1 - TRUE or 0 - FALSE) and the cpu value in an array ref. Sorting can then be done on the indicator first then on cpu value but the second sort term is a ternary so that comparing two items with a "-" cpu value immediately returns zero.

$ perl -Mstrict -Mwarnings -E '
my $passes = {
   t1 => { cpu => 73, xx => 3 },
   t2 => { cpu => q{-}, xx => 7 },
   t3 => { cpu => 17, xx => 0 },
   t4 => { cpu => 49, xx => 6 },
   t5 => { cpu => q{-}, xx => 4 },
   t6 => { cpu => 11, xx => 5 },
   };

say for
   map  { $_->[ 0 ] }
   sort {
           $a->[ 1 ] <=> $b->[ 1 ]
           ||
           ( $a->[ 1 ] ? 0 : $a->[ 2 ] <=> $b->[ 2 ] )
        }
   map  { [
           $_,
           $passes->{ $_ }->{ cpu } eq q{-},
           $passes->{ $_ }->{ cpu },
          ]
        }
   keys %$passes;'
t6
t3
t4
t1
t2
t5
$
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I hope this is of interest.

Update: Ignore this, it doesn't seem to be sorting correctly :-(

Update 2: I was bitten on the bum by a silly precedence problem that should have occurred to me sooner. Replacing

   sort {
           $a->[ 1 ] <=> $b->[ 1 ]
           ||
           $a->[ 1 ] ? 0 : $a->[ 2 ] <=> $b->[ 2 ]
        }
[download]

with

   sort {
           $a->[ 1 ] <=> $b->[ 1 ]
           ||
           ( $a->[ 1 ] ? 0 : $a->[ 2 ] <=> $b->[ 2 ] )
        }
[download]

solved the problem.