Hello powerman, I also would like to know the solution for this...

$_ would be used with interpolated string, like die "open $file failed $!", So, I would like to catch the timing for decode with tie variable or commandline option or hopefully, similar functionality like IO layer.

I tried with tie. This seems working. So, I guess changing every $! variable will become adding 2 lines to the head of script.

use DecodedMsg;
tie $! , "DecodedMsg";
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Below is my tests. But I have to confess, I am not familiar with theses "tie" things. Hope comments of glues.

use strict; use warnings;
use Encode qw(decode encode);

sub show{
    printf "is_utf8=%d, len=%d, msg=%s\n", utf8::is_utf8($_), length($
+_), $_; 
}

#byte string utf8 hiragana
my $byte_error_msg=encode('UTF-8', join('', map{pack('U',$_)} 0x3042 .
+. 0x3052));

print "#test1: with byets\n";
$_ = $byte_error_msg ; 
show;

print "#test2: with decoded char\n";
$_ = decode("UTF-8", $byte_error_msg);
show;

print "#test3 with tied variable ... seems working\n";
{package DecodedMsg;
    use Encode qw(decode);
    sub TIESCALAR { 
        my $class=shift;
        print "class=$class\n";
        return bless( {} , $class );
    }
    sub STORE{ 
        my $self=shift;
        print "###in store arg=$_[0]\n";
        $self->{val}= $_[0];
    }    
    sub FETCH { 
        my $self=shift;
        if( utf8::is_utf8( $self->{val} ) ){
            return  $self->{val};
        } else {    
            print "#in fetch ... return decoded\n";
            return     decode('UTF-8', $self->{val})
        }
    }
    1;
}

tie $_, "DecodedMsg";    #tie $_ to DecodedMsg
$_ = $byte_error_msg;
show;

print "#test4 with tied variable ... with system error\n";
tie $!, "DecodedMsg";
open(my $fh, "<", " /the/path/to/nowhere");
$_ =$!;
show;
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