Assuming that you need to change 'ABCD' into '\AB\CD', there should be an easier way:

sub enc_hex {
  my $out = unpack('H*', (shift));
  $out =~ s/\G(..)/\\$1/g;
  return $out;
}
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This uses the regex engine to advance through the string two characters at a time, inserting backslashes before each pair in the output. See perlre and perlop for more details.