[dean:~]$ perl -v

This is perl, v5.8.1-RC3 built for darwin-thread-multi-2level
(with 1 registered patch, see perl -V for more detail)

Copyright 1987-2003, Larry Wall
...

[dean:~]$ cat /tmp/deleteme.pl 
#!/usr/bin/perl
use strict;
use warnings;

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

[dean:~]$ /tmp/deleteme.pl 'a\Ubc'
Unrecognized escape \U passed through in regex; marked by <-- HERE in m/a\U <-- HERE bc/ at /tmp/deleteme.pl line 6.
'aBC' =~ (?-xism:a\Ubc)


##</code><code>##

#!/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 $/;