Some sort of deep doodoo in the XS voodoo. Perhaps the following code?

# Call.XS ~line 130
    SAVESPTR(DEFSV) ;/* save $_ */
    /* make $_ use our buffer */
    DEFSV = sv_2mortal(newSVpv("", 0)) ;
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A maybe workaround is to quote uses of $_, which seems to prevent the bug:

use Filter::Util::Call;
BEGIN{ filter_add(sub{
        my $s = filter_read;
        my $t = "$_";
        $s;
}); }
print "hello, world\n";
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