The & is the sigil for code. &foo is for subroutines what $foo is for scalars and @foo is for arrays and so on. &POSIX::O_RDWR is probably defined in package POSIX as a sub O_RDWR { ... }

In Perl5, the & does not necessarily take a reference to the subroutine; it could be calling the routine, depending on several things. In Perl6 this will get cleared up, and the & sigil will always take a coderef. 

$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/

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In reply to Re: Ampersand prefix for package constants ? by jonadab
in thread Ampersand prefix for package constants ? by Anonymous Monk

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