As noted before, you don't need to encode each character. Also, you can build a regex that matches all the keys of the hash, than you don't need to call any subroutine from /e. This way, you only replace the characters you know how to replace, so it will work even for the $ which is ASCII 36.

my $chars = join "", keys %_table;
sub escapeUTF8  # Now needs a better name!
{
   my ( $sgml_r) = @_;
   $$sgml_r =~ s/([\Q$chars\E])/$_table{$1}/g;
}
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