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It is indeed a missing SvSETMAGIC.

use strict; use warnings; use Devel::Peek qw( Dump ); use Inline C => <<'__EOI__'; void buggy_assign(SV* dsv, SV* ssv) { SvSetSV_nosteal(dsv, ssv); /* Should call SvSetMagicSV_nosteal(dsv, ssv); instead * or should call SvSETMAGIC(dsv); afterwards. */ } void set_magic(SV* sv) { SvSETMAGIC(sv); } __EOI__ my $txt_de = "K\N{U+00E4}se"; my $txt_ru = "\N{U+041C}\N{U+043E}\N{U+0441}\N{U+043A}\N{U+0432}\N{U+0 +430}"; # \N should have done this already. utf8::upgrade($_) for $txt_de, $txt_ru; my $txt; buggy_assign($txt, $txt_de); print(length($txt), "\n"); Dump($txt); buggy_assign($txt, $txt_ru); print(length($txt), "\n"); Dump($txt); set_magic($txt); print(length($txt), "\n"); Dump($txt);
4 SV = PVMG(0x81af530) at 0x817bca0 REFCNT = 1 FLAGS = (PADMY,SMG,POK,pPOK,UTF8) IV = 0 NV = 0 PV = 0x8177b98 "K\303\244se"\0 [UTF8 "K\x{e4}se"] CUR = 5 LEN = 8 MAGIC = 0x82f0d98 MG_VIRTUAL = &PL_vtbl_utf8 MG_TYPE = PERL_MAGIC_utf8(w) MG_LEN = 4 4 SV = PVMG(0x81af530) at 0x817bca0 REFCNT = 1 FLAGS = (PADMY,SMG,POK,pPOK,UTF8) IV = 0 NV = 0 PV = 0x844b718 "\320\234\320\276\321\201\320\272\320\262\320\260"\0 +[UTF8 "\x{41c}\x{43e}\x{441}\x{43a}\x{432}\x{430}"] CUR = 12 LEN = 16 MAGIC = 0x82f0d98 MG_VIRTUAL = &PL_vtbl_utf8 MG_TYPE = PERL_MAGIC_utf8(w) MG_LEN = 4 6 SV = PVMG(0x81af530) at 0x817bca0 REFCNT = 1 FLAGS = (PADMY,SMG,POK,pPOK,UTF8) IV = 0 NV = 0 PV = 0x844b718 "\320\234\320\276\321\201\320\272\320\262\320\260"\0 +[UTF8 "\x{41c}\x{43e}\x{441}\x{43a}\x{432}\x{430}"] CUR = 12 LEN = 16 MAGIC = 0x82f0d98 MG_VIRTUAL = &PL_vtbl_utf8 MG_TYPE = PERL_MAGIC_utf8(w) MG_LEN = 6

The first one works because length hasn't placed the magic yet.

SV = PV(0x816a0c0) at 0x817bca0 REFCNT = 1 FLAGS = (PADMY,POK,pPOK,UTF8) PV = 0x8177b98 "K\303\244se"\0 [UTF8 "K\x{e4}se"] CUR = 5 LEN = 8

Assigning to $txt — your workaround — works because assignment properly calls SvSETMAGIC, which clears the precomputed length of the string, thus forcing the next call to length to recalculate it.

SV = PVMG(0x81af570) at 0x817bca0 REFCNT = 1 FLAGS = (PADMY,SMG,POK,pPOK) IV = 0 NV = 0 PV = 0x8177b98 ""\0 CUR = 0 LEN = 8 MAGIC = 0x831d6a0 MG_VIRTUAL = &PL_vtbl_utf8 MG_TYPE = PERL_MAGIC_utf8(w) MG_LEN = -1

In reply to Re^5: length() returns wrong result - suspicious magic by ikegami
in thread length() returns wrong result - suspicious magic by mje

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