pc88mxer has already answered your specific questions. I thought I'd give you some tips to make using regexps a little easier on the eyes.

1. You can create regexp objects with the qr operator. This allows you to build more complex regexps out of simple pieces, and to give the individual pieces names.
2. Inside character classes, only ^ and - are special. There's no need to escape the rest, which removes a lot of your backslashes.

Example:

my $drivespec = qr{[a-zA-Z]:};
my $dir            = qr{[^<>":|?*]*};
my $path         = qr{^ $drivespec? $dir $}x;

unless( $input->{$key} =~ $path ) {
    ...
}
[download]

This barely scratches the surface, of course. More reading at perlrequick, perlretut, perlre, and here in the Tutorials section.