http://www.perlmonks.org?node_id=156912

Choosing a username

There are no hard and fast rules for choosing a username, other than that it must be unique and should be relatively easy to type. Taking a few minutes to choose a good username is worthwhile, since your username can't be changed later, and is used in many ways such as accessing your scratchpad, sending you messages, and referring to you in posts. Choose wisely.

Consider that your username is one of the first things other Perl Monks will know about you. What sort of a message do you wish to send? Advice you offer with the username "CompleteMoron" may not be taken too seriously! Likewise, choosing "PerlNewbie" as your username is also ill advised, unless you plan on remaining a newbie forever. People don't name their newborn babies "Born Yesterday". They pick names which will last a lifetime. Give yourself a little credit; you won't always be a newbie, will you?

Memorability and Typability Considerations

How easy is your username to remember and type? When people want to send you private messages, or address you or mention you in the chatterbox or in postings, they will have to type out your username. For this reason, usernames should not be too long, and should avoid punctuation and other odd characters HaX0r-$TyL3 names are also difficult for people to remember and type — and won't impress the Monks anyway. So take a few minutes to choose your username carefully, and avoid potential frustrations later.

Restrictions

Some names are "reserved". The following cannot be used as usernames:

Checking for such collisions is performed case-insensitively, and space and underscore are treated as equivalent.

Also, there are some technical restrictions on valid usernames, including (but not limited to) the following:

If there is a problem with the name you're trying to register, PerlMonks will let you know, and ask you to select a different name.

to the PerlMonks FAQ