use constant FOO => 42;

##</code><code>##

sub FOO { 42 }

##</code><code>##

use POSIX ();

##</code><code>##

$ perl -mPOSIX -e '$a = POSIX::O_RDWR; print "$a\n"'
POSIX::O_RDWR
$ perl -mPOSIX -e '$a = &POSIX::O_RDWR; print "$a\n"'
2

##</code><code>##

use POSIX qw(O_RDWR);

##</code><code>##

$ perl -mPOSIX=O_RDWR -e '$a = O_RDWR; print "$a\n"'
2