>perl -wMstrict -le
"my $chr;
 $chr = \"x this and that y\";
 $chr =~ s/(this|that)|(\w+)/$1\U$2/g;
 print \"$chr\n\";
"
Use of uninitialized value in concatenation (.) or string ...
Use of uninitialized value in concatenation (.) or string ...
Use of uninitialized value in concatenation (.) or string ...
X this AND that Y

##</code><code>##

>perl -wMstrict -le
"my $chr;
 $chr = \"x this and that y\";
 $chr =~ s/(this|that)|(\w+)/\u$1\E$2/g;
 print \"$chr\n\";
"
Use of uninitialized value in concatenation (.) or string ...
Use of uninitialized value in concatenation (.) or string ...
x This and That y

##</code><code>##

>perl -wMstrict -le
"my $chr;
 $chr = \"x this and that y\";
 $chr =~ s/(this|that)|(\w+)/\u$1\E\U$2/g;
 print \"$chr\n\";
"
X This AND That Y