Create a function that calls your function with the correct argument, and pass a reference to that function instead.

find(sub { findNewerThan(days(5)) }, $file_dir_root);
[download]

or

my $days = days(5);
find(sub { findNewerThan($days) }, $file_dir_root);
[download]

Update: I accidently changed days(5) into 5. Fixed.