Nobody else seems to have pointed this out, so I will..

foreach $a(@letters)
[download]

Although I realise this is just a short code snippet, the use of $a as a variable name is a bad habit(tm) to get into. Both $a and $b are package globals which are used by sort. If you get into the habit of using them elsewhere, this habit will eventually come back and bite you on the rear end ;)

Cheers,
Darren :)