#!/usr/bin/perl
use strict;
use warnings;

no warnings "uninitialized";

my $patt = 'a\Ubc';
$patt =~ s/(^|[^\\])(\\\\)*\\U(.*?)(?:\\E|$)/$1.$2.uc($3)/se;
$patt = qr/$patt/;

my $target = 'aBC';
print "'$target' =~ $patt\n";
print $target =~ $patt;
print $/;