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s/(old)/preserve_case("new", $1)/ieg;

...
  my $mask = $from & (' ' x length($from));
  return uc($to) | $mask;
}

# requires $" hasn't been altered
#       123456789_123456789_12345678 = 28
sub pc {uc$_[0]|$_[1]&$"x length pop}

sub preserve_case {
  my ($to, $from) = @_;
...
  my $mask = join '', map /\w/ ? $_ & ' ' : "\0", split //, $from;
  return uc($to) | $mask;
}

(my $mask = $from) =~ s/(\w)|./ $1 ? $1 & ' ' : "\0" /egs;

# requires $" hasn't been altered
#       123456789_123456789_123456789_123456789_123456789_ = 50
sub pc {(my$x=pop)=~s/(\w)|./$1?$1&' ':"\0"/egs;uc$_[0]|$x}

sub preserve_case {
  my ($to, $from) = @_;
...

  return uc($to) | $mask;
}

sub preserve_case {
  my ($to, $from) = @_;
...

  return uc($to) | ($from ^ uc $from);
}