seek DATA, -53, 1; eval <DATA> or print "Not Windows\n" or print "Windows\n"; __DATA__

eval will execute either
print "Not Windows\n" or print "Windows\n"; or
int "Not Windows\n" or print "Windows\n";
based on the line ending used in the source.

Keep in mind that print returns true on success.

Inspired by jagh's kinda japhy.