Before you fixed your typo, the second binmode in the second snippet was failing, so a second encoding layer wasn't being added, so you didn't double-encode.

With the typo fixed, you add two encoding layers, so you double-encode the second print, so you get garbage for the second print.

With typo fixed:

$ perl b1.pl
中文
"\x{0590}" does not map to cp936 at b1.pl line 8.
"\x{0384}" does not map to cp936 at b1.pl line 8.
\x{0590}\x{0384}

$ perl b2.pl
中文
"\x{0590}" does not map to cp936 at b2.pl line 7.
"\x{0384}" does not map to cp936 at b2.pl line 7.
\x{0590}\x{0384}
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