As another Anonymous Monk pointed above you can use warnings 'FATAIL' for all warnings or only "uninitialized". Lexical scope used. But this will catch other types of "uninitialized" warnings.
in reply to Re^2: Turn matching against undef to be deadly
in thread Turn matching against undef to be deadly
use warnings FATAL => 'uninitialized';
printf "matched: %s\n"
, ( 'x' =~ undef() ? 'yes' : 'no' )