>perl -wMstrict -le
"for (@ARGV) {
   if(/^A (?: Z | (\s.*?)) Z$/x) {
     my $grabbed = $1 // '';
     print qq{matched '$_'  grabbed '$grabbed'};
     }
   }
"  AZ  AZZ  AXZ  "A SOMETHING Z"  ASOMETHINGZ
matched 'AZZ'  grabbed ''
matched 'A SOMETHING Z'  grabbed ' SOMETHING '


##</code><code>##

>perl -wMstrict -le
"for (@ARGV) {
   if(/^A ((?: \s [^Z]*)?) Z$/x) {
     print qq{matched '$_'  grabbed '$1'};
     }
   }
"  AZ  AZZ  AXZ  "A ZZ"  "A SOMETHING Z"  ASOMETHINGZ  "A Z"  "A  Z"
matched 'AZ'  grabbed ''
matched 'A SOMETHING Z'  grabbed ' SOMETHING '
matched 'A Z'  grabbed ' '
matched 'A  Z'  grabbed '  '