You can pass a filehandle to write_file:
use File::Slurp qw(write_file);

my $ustr = "simple unicode string \x{0434} indeed";

{
    open(my $FH, ">:encoding(UTF-8)", "a.txt")
      or die "Failed to open file - $!";

    write_file($FH,  $ustr)
      or warn "Failed write_file";
}

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In reply to Re: can not use utf8 with File::Slurp by repellent
in thread can not use utf8 with File::Slurp by vkon

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