Why capture when you don't have to?

$zipfile =~ /(?:A|B)FILE_(\d+)/;
$day = $1;
[download]

Or you can use a character class (since the strings are 1 char long).

$zipfile =~ /[AB]FILE_(\d+)/;
$day = $1;
[download]

Of course, you could always use the uber-sexy:

($day) = $zipfile =~ /[AB]FILE_(\d+)/;
[download]

;-P

Anonymously yours,
Anonymous Nun