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PerlIOBase_read(pTHX_ PerlIO *f, void *vbuf, Size_t count)
{
...
            return 0;
        }
        ...

#!/usr/bin/perl

...
    PerlIO_modestr(f, buf);
    fprintf(stderr, "mode = %s\n", buf);
}

$ ./802590.pl 0
mode = w
$ ./802590.pl 1     # with explicit close
mode = r+

$ PERLIO_DEBUG=/dev/tty ./802517.pl 1    # with explicit close
...
...
./802517.pl:15 PerlIO_push f=0x6253c0 unix w+ 0x603b08
./802517.pl:15 fd 1 refcnt=1
./802517.pl:15 PerlIO_push f=0x6253c0 perlio w+ 0x603b08

$ PERLIO_DEBUG=/dev/tty ./802517.pl 0
...
...
./802517.pl:15 fd 8 refcnt=0
./802517.pl:15 PerlIO_pop f=0x6253e8 perlio
./802517.pl:15 PerlIO_pop f=0x6253e8 unix