tr is the one that does it: 
my $r = "SomEThingWithMIXEDRegiSter";
#$r =~ tr/[A-Z][a-z]/UUUUUUUUUUUUUUUUUUUUUUUUUULLLLLLLLLLLLLLLLLLLLLLL
+LLL/;
eval( '$r =~ tr/[A-Z][a-z]/'.('U' x 26).('L' x 26).'/' );
print $r;

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The string in eval expands to the commented line above it.
For every complex problem there is an answer that is clear, simple, and wrong. H. L. Mencken

In reply to Re: change [A-Z] to U, [a-z] to L with one regex? by hdb
in thread change [A-Z] to U, [a-z] to L with one regex? by Doctrin

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