Firstly, you would want PREREQ_PM in the WriteMakefile() section of the Makefile.PL to include Win32::Fmode if and only if $^O =~ /mswin32/i. Then near the beginning of the module (.pm file) you can have something like:

if($^O =~ /mswin32/i) {require Win32::Fmode}
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You can then access the Win32::Fmode functions by their fully qualified names. Whenever you wish to determine the read/write status of a filehandle you can do something like:

if($^O =~ /mswin32/i) {
  # employ the Win32::Fmode functions
  }
else {
  # employ the Fcntl flags/macro
  }
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Cheers,
Rob