It's not clear to me how you're getting the results you describe. There are several problems with your code, however.

The principal one is that you never use your crypted $passc. What you're saving is the clear text. The file you save it to should not be opened with '>' mode, but '>>', or, better, '+<' so you can read the file and check for duplicated user names. File locking would also be wise.

You can call next in the read loop if the $user doesn't match, and last out of the loop once you find the entry for $user and compare crypts. That will let you avoid some unnecessary work.

chomp before split and after reading from STDIN to remove line ends which will wreck comparisons.