Another approach (assumption: string is one byte per character):

>perl -wMstrict -le
"for my $in (@ARGV) {
   my @nuxi =
     map sprintf('%04x', $_),
     unpack 'v*', $in
     ;
 print qq{$in -> @nuxi};
}
" test tester teste
test -> 6574 7473
tester -> 6574 7473 7265
teste -> 6574 7473
[download]

Note that this approach fails for odd-length strings. Depending on just what output you want, this can be fixed, e.g.:

>perl -wMstrict -le
"for my $in (@ARGV) {
   my @nuxi =
     map sprintf('%04x', $_),
     unpack 'v*', $in . qq{\000}
     ;
 print qq{$in -> @nuxi};
}
" test tester teste
test -> 6574 7473
tester -> 6574 7473 7265
teste -> 6574 7473 0065
[download]

(This is an example of the NUXI problem.)