Thank you very much, Your Mother!

This works wonderfully, and it makes sense given the known cause of the encoding corruption.

use v5.16;
use utf8;
use open qw( :encoding(UTF-8) :std );
use Encode qw( encode decode );

while (my $damaged_text = <DATA>) {
    chomp $damaged_text;

    my $repaired_text = decode('UTF-8', encode('UCS-2LE', $damaged_text));

    say $repaired_text;
}

close DATA;

exit 0;

__DATA__
敒›剕䕇呎
敌馀⁳潧琠韦겜鯥↽

This prints…

    Re: URGENT
    Let’s go to 日本国!

…as expected.

In this case, use utf8 is required.