Calling subroutines with an ampersand but without parentheses is a special invocations that preserves @_ and should only be used in special advanced situations. This is not one of those.

You'd be better off writing POSIX::O_RDWR() or (if you used strict) POSIX::O_RDWR. Some information related to such choices is summarized in (tye)Re: A question of style.

Update: For those players at home confused by this answer (thinking I got my advice backward as far as strict.pm is concerned, one of whom was kind enough to /tell me about it)... The point of strict.pm is pretty much to catch simple mistakes, including typos.

Let's take two simple mistakes as examples: 1) you mispelt "O_RDWR" or 2) forgot to "use POSIX". Now consider these correct usages:

perl -le "            use POSIX; print 'x',POSIX::O_RDWR();"
perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWR();"
perl -le "            use POSIX; print 'x',POSIX::O_RDWR;"
perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWR;"
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which gives us results similar to the following:

 > perl -le "            use POSIX; print 'x',POSIX::O_RDWR();"
x2
 > perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWR();"
x2
 > perl -le "            use POSIX; print 'x',POSIX::O_RDWR;"
x2
 > perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWR;"
x2
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Now consider these mistakes:

perl -le "                       print 'x',POSIX::O_RDWR();"
perl -le "            use POSIX; print 'x',POSIX::O_RDWT();"
perl -le "use strict;            print 'x',POSIX::O_RDWR();"
perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWT();"
perl -le "use strict;            print 'x',POSIX::O_RDWR;"
perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWT;"
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which are all caught for us as follows:

 > perl -le "                       print 'x',POSIX::O_RDWR();"
Undefined subroutine &POSIX::O_RDWR called at -e line 1.
 > perl -le "            use POSIX; print 'x',POSIX::O_RDWT();"
O_RDWT is not a valid POSIX macro at -e line 1
 > perl -le "use strict;            print 'x',POSIX::O_RDWR();"
Undefined subroutine &POSIX::O_RDWR called at -e line 1.
 > perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWT();"
O_RDWT is not a valid POSIX macro at -e line 1
 > perl -le "use strict;            print 'x',POSIX::O_RDWR;"
Bareword "POSIX::O_RDWR" not allowed while "strict subs" in use at -e 
+line 1.
Execution of -e aborted due to compilation errors.
 > perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWT;"
Bareword "POSIX::O_RDWT" not allowed while "strict subs" in use at -e 
+line 1.
Execution of -e aborted due to compilation errors.
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and finally consider these mistakes:

perl -le "                       print 'x',POSIX::O_RDWR;"
perl -le "            use POSIX; print 'x',POSIX::O_RDWT;"
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which are not caught for us:

 > perl -le "                       print 'x',POSIX::O_RDWR;"
xPOSIX::O_RDWR
 > perl -le "            use POSIX; print 'x',POSIX::O_RDWT;"
xPOSIX::O_RDWT
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and you see why I say you should only write POSIX::O_RDWR if you used strict (because if you try to write POSIX::O_RDWR without using strict.pm, you aren't protected from a simple mistake going unreported).

                - tye