I tried this and is working. Thank you. However, when i use -e to test it, seems not working. Could you pointed me out?

 
if (-e m{ \A $input some-fixed-string .*? [.]pdf \z }x}
..
}
[download]

You are not checking for existence of a file path, but the result of regex match against $_.

I think you would be better off (for some definition of "better") with Path::* solution suggested earlier instead of rolling a solution on your own. Also, please do the RTFM.