package Old::MyModule;

my $contents;
foreach my $prefix (@INC) {
    my $file = "$prefix/New/MyModule.pm";
    if(-f $file) {
        open(FILE, "<", "$file") or die "$file: $!";
        $contents = do { local $/; <FILE> };
        close(FILE);
        last;
    }
}

die "Can't locate New::MyModule in \@INC (\@INC contains: @INC)." unless($contents);
$contents =~ s/package New::MyModule/package Old::MyModule/;
my $result = eval $contents;
die "$file: $@" if($@);
die "$file: did not end with a true value" unless($result);

1;