Why not just do the reverse when requested?

my @ordered = sort @array;
@ordered = reverse @array if $something;
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You could use eval STRING, but that's evil, so let's avoid that. Another possibility is:

foreach ($something ? reverse sort @array : sort @array)
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but I don't really like that - too much copying (especially if you have a sort function). Another one that is close to being nice is:

use Sort::Key qw(keysort rkeysort);
my $sorter = $something ? \&rkeysort : \&keysort;
foreach ($sorter->(sub { $_ }, @array) )
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(Untested - you may need to pass in \@array.) Along these lines, you could use the multikeysorter from Sort::Key) to create a sorter sub that sorts in the correct order - you pass in something based on the $something.