For reading input, using ":encoding(utf8)" instead of bare ":utf8" is strongly recommended.

While this is correct (for security reasons), it's unlikely to help with the OP's (presumed) problem of getting rid of a BOM in the input data. In other words, :encoding(utf8) (just like :utf8) does not filter out the BOM:

my $file = "somefile.utf8";

# create a UTF-8 encoded test file, explicitly adding a BOM
open my $out, ">:utf8", $file or die $!;
print $out "\x{feff}foo bär";
close $out;

# read it back in
open my $in, "<:encoding(utf8)", $file or die $!;
$_ = <$in>;
use Devel::Peek;
Dump $_;
[download]

SV = PV(0x793cd0) at 0x7c53e0
  REFCNT = 1
  FLAGS = (POK,pPOK,UTF8)
  PV = 0x7c9088 "\357\273\277foo b\303\244r"\0 [UTF8 "\x{feff}foo b\x{
+e4}r"]
  CUR = 11                                               ^^^^
  LEN = 80
[download]