Did you show the entire output of 'perl -V' ?
Usually the current working directory ('.') is in @INC, and if '/home/donghe/iprscan/lib' is your current working directory, then '/home/donghe/iprscan/lib/IO/Scalar.pm' would be found by use IO::Scalar;. (Note that the @INC, as reported by the error message *does* include '.')

However, it can never make any sense that '/home/donghe/iprscan/lib/Scalar.pm' would be found by attempting to use IO::Scalar;

And whatever happened to the perl that's in /opt (referenced in your original post) ?

Cheers,
Rob