There are multiple ways to access the array elements. The most direct would be:

my %files = (
    dev    => [qw(dev_file1 dev_file2 dev_file3 dev_file4)],
    prod   => [qw(prod_file1 prod_file2 prod_file3 prod_file4)],
    report => [qw(report_file1 report_file2 report_file3 report_file4)
+],
);

print $files{dev}[0];
[download]

or you could do this

my @dev = @{ $files{dev} };
print $dev[0];
[download]