Quite simply, this Perl program converts the text you give it into HTML entities. Not much else to it, really, so I'll just give an example:
$ text2htmlent.pl Enter item to change: my@email.address my@email.address
Of course you don't have to use it for encoding e-mail addresses (I only do this for obfuscation purposes), you can use it to transform any ASCII character into an HTML entity. And yes, I realise this sort of thing has probably been done hundreds of times, but it was something I needed quickly and couldn't be bothered hunting around, so took 5 mins to write this.