In line 17 you open a file. The filehandle is stored in a typeglob called READFILE, and the file's name is stored in the scalar variable $infile.

In line 19 you attempt to read from a filehandle held in $infile. But it's not intended to be used as a filehandle; it's just a string holding the name of a file that the user entered in line 8. You should change line 19 to: while(<READFILE>){.