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.

use strict;
use warnings;
use Carp;
use warnings FATAL => 'uninitialized';
printf "matched: %s\n"
  , ( 'x' =~ undef() ? 'yes' : 'no' )
    ;

print "end\n";
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