... a way around the problem ...
...
use lib "path_to_perl5" if $OS == 'MSWin32'

Others have correctly commented that the problem described in the OP is much more basic than conditional compilation (update: and seems to be essentially a system configuration problem). In addition to | Be that as it may, beyond the comparison operator problem pointed out by Tux, a use statement is executed at compile-time, but a
    if (condition) { ... }
or
    ... ifcondition;
statement executes at run-time. So the effect of

c:\@Work\Perl\monks>perl -wMstrict -le
"if ('night' eq 'day') {
   use constant FOO => 42;
   }
 ;;
 print FOO;
"
42
[download]

is not what you expect.

Consider the if pragma for use in cases like this — but not in this particular case!

c:\@Work\Perl\monks>perl -wMstrict -le
"use if 'night' eq 'day', constant => FOO => 42;
 ;;
 print FOO;
"
Name "main::FOO" used only once: possible typo at -e line 1.
print() on unopened filehandle FOO at -e line 1.
[download]

Update: Made some changes in the first paragraph to try to clarify the intent and focus of this reply.