use strict;
no warnings 'redefine';
use Encode;
use Data::Dumper;
our $REAL;
BEGIN {
   $REAL = \&Encode::decode_utf8;
}

sub Encode::decode_utf8 ($;$) {
   my $hash = shift;
   for (keys %{$hash}) {
      $hash->{$_} = $REAL->($hash->{$_})
   }
   return $hash;
}

my $ref = Encode::decode_utf8({ foo => 1});
print Dumper($ref);