In Perl 5, constants _are_ subroutines:

use constant FOO => 42;
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is basically syntactic sugar for:

sub FOO { 42 }
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Normally, you wouldn't need to use the & sigil, but many of the POSIX constants are basically AUTOLOADed subroutines. So I bet you have something like:

use POSIX ();
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at the start of your program. Because the subroutines have not been encountered at compile time, the compiler needs a hint that it is a subroutine (otherwise it will assume it is a "bareword"). Observe:

$ perl -mPOSIX -e '$a = POSIX::O_RDWR; print "$a\n"'
POSIX::O_RDWR
$ perl -mPOSIX -e '$a = &POSIX::O_RDWR; print "$a\n"'
2
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An alternate way is to export the constant your own namespace so that you can refer to it directly without the POSIX:: prefix:

use POSIX qw(O_RDWR);
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or, as a one liner:

$ perl -mPOSIX=O_RDWR -e '$a = O_RDWR; print "$a\n"'
2
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Hope this helps.

Liz