For me

unless ($expression){
  # do something
}
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is never elegant. I'll always plump for

if (not $expression){
 # do something
}
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especially if, as in this case $expression includes some boolean arithmetic. I do use it for early exits from loops though. I can read

while (<$fh>){
 next unless /\S/;
 #
}
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without too much trouble (as long as it's simple).

As soon as an expression has a not an and and an or in it I'm almost sure to get it wrong and it often points to a poor algorithm which some refactoring can fix. Either that or put it into sub so I can say

if (work_to_do($args)){
 # do some work
}
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and test the living daylights out of it.

Did I mention I've had awful trouble with boolean arithmetic? :-)