foreach my $zipfile (@files) {
    my $day = undef;
    $zipfile  =~ m/AFILE_(\d+)/;
    $zipfile  =~ m/BFILE_(\d+)/;
    $day = $1;
}
[download]

That code isn't correct. If @files contains for instance "AFILE_foo.zip", neither of the regexes matches, and $1 will still be the result of the previous successful match.

Abigail