>perl -wMstrict -le
"func('before');
 my $tricky = 'initialized at runtime';
 func('after');
 ;;
 sub func {
   printf qq{$_[0]: %s defined: }, defined($tricky) ? 'is' : 'NOT';
   print  qq{'$tricky'};
   }
"
Use of uninitialized value $tricky in concatenation (.) or string ...
before: NOT defined: ''
after: is defined: 'initialized at runtime'