Thats a FAQ and IMHO 99% of the cases of assignments to symbolic references in Perl are a result of weak code design (sorry!)

the following works under no strict 'refs'

  DB<109> $var="var";$id=0
 => 0

  DB<110> ${$var.$id}=42
 => 42

  DB<111> $var0
 => 42
[download]

Are you sure you need that? Be warned about the consequences!

Older discussions regarding 'no strict refs' might help you getting an insight why.

edit

Are you aware of the concept of hashes?