Forget that for a moment, does it work if you add the flags?

I should have been more specific. It's not allowed in the sense that the script will die with an error message "too late to add -C flag".

oh, right, forgot about that

What about the Dumper?

If i try

perl -CSD -le " print qq{use utf8;\nprint qq{I \x{2665} Perl\n};\n}; "
+ >foo.pl
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I get

use utf8;
print qq{I ♥ Perl
};

If I run that through perltidy or your program its unchanged

cmd.exe doesn't know how to display the unicode without a chcp, but the bytes are the same

If I add to your program

    use Data::Dump qw/pp /;
    print STDERR pp("$output_string");
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I get unchanged correct result as expected

"use utf8;\nprint qq{I \xE2\x99\xA5 Perl\n};\n\n"
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by default cmd.exe does not unicode

$ chcp
Active code page: 437

$ type foo.tdy
use utf8;
print qq{I ΓΦΡ Perl
};

If I change it I get a heart

$ chcp 65001
Active code page: 65001

$ type foo.tdy
use utf8;
print qq{I ♥ Perl
};

I must be misunderstanding something, but I don't know what you're suggesting I do to actually make my code work.

How would I use that to make a replacement for my Perl Tidy utility?