Changing your code to follow my earlier suggestion would give something like this:

my $cssFiles = [];
push @{$cssFiles}, $_css_path;
push @{$cssFiles}, $_css_path2;
# Then in the start_html() this line:
-style => { -code => $page_style, -src => $cssFiles },
[download]

If you wanted to use an array instead of an array ref, you could try this:

my @cssFiles;
push @cssFiles, $_css_path;
push @cssFiles, $_css_path2;
# Then in the start_html() this line:
-style => { -code => $page_style, -src => \@cssFiles },
[download]