use strict;
use warnings;
use Carp;


local $SIG{__WARN__} = sub {
if ($_[0] =~ /Use of uninitialized value in regexp compilation/) {
confess $_[0];
} else {
 warn @_;
}
};

print "x".undef, "\n";

printf "matched: %s\n"
  , ( 'x' =~ undef() ? 'yes' : 'no' )
    ;
__END__
Use of uninitialized value in concatenation (.) or string at 1.pl line 14.
x
Use of uninitialized value in regexp compilation at 1.pl line 16.
 at 1.pl line 7.
        main::__ANON__('Use of uninitialized value in regexp compilation at 1.pl line...') called at 1.pl line 16