:utf8 doesn't fix bad data. 

$ perl -MDevel::Peek -we'my $buf = "\x80"; open my $fh, "<:utf8", \$bu
+f or die; my $x = <$fh>; Dump $x;'
utf8 "\x80" does not map to Unicode at -e line 1, <$fh> line 1.
SV = PV(0x814fbb4) at 0x814f6cc
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK,UTF8)
  PV = 0x8170a78 "\200"\0Malformed UTF-8 character (unexpected continu
+ation byte 0x80, with no preceding start byte) in subroutine entry at
+ -e line 1, <$fh> line 1.
 [UTF8 "\x{0}"]
  CUR = 1
  LEN = 80

[download]

:encoding(UTF-8) replaces the bad data (with the 4 chars '\x80' in this case). 

$ perl -MDevel::Peek -we'my $buf = "\x80"; open my $fh, "<:encoding(UT
+F-8)", \$buf or die; my $x = <$fh>; Dump $x;'
utf8 "\x80" does not map to Unicode at -e line 1.
SV = PV(0x814fbb4) at 0x814f6cc
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK,UTF8)
  PV = 0x8197048 "\\x80"\0 [UTF8 "\\x80"]
  CUR = 4
  LEN = 80

[download]

I can't answer your other questions.

In reply to Re: :utf8 I/O layer vs encoding(UTF8), segfault and speed by ikegami
in thread :utf8 I/O layer vs encoding(UTF8), segfault and speed by mje

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