As ikegami pointed out, /g in scalar context
doesn't reset pos. Your example will work fine
after some small modifications, eg.:

 ...
  sub func
 {
  print 'The Command Seperator is: ', $^O =~ /MSWin32/i ? '&' : ';', "
+\n"
 }

 func();
 func();
 ...
[download]

<Addendum>
After thinking about an idiomatic switch between alternatives
I got this one, which I like somehow:

  sub func
 {
  print 'The Command Seperator is: ',
         qw(; &)[ $^O =~ /MSWin32/i ],
         "\n"
 }
[download]

which evaluates the regex in scalar context as 0/1 (array index of the qw...)
</Addendum>

Regards

mwa