In your first example, you did a use POSIX; in the second, you did not. Why?

Without loading POSIX you won't be able to call a function like POSIX::cuserid.

Enable strictwarnings and see yourself what happens: 
$ perl -wE 'say POSIX::cuserid'
Name "POSIX::cuserid" used only once: possible typo at -e line 1.
say() on unopened filehandle cuserid at -e line 1.

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But with loading POSIX: 
$ perl -MPOSIX -wE 'say POSIX::cuserid'
linuxer

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Updates

In reply to Re: Diff between 'print POSIX::cuserid' and 'use POSIX; print cuserid' by linuxer
in thread Diff between 'print POSIX::cuserid' and 'use POSIX; print cuserid' by wsanders

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