It's not a bug in Perl, and it's not a bug in File::Spec::Functions: it's documented (and intended) behavior.

At the beginning of File::Spec, it looks at the value of $^O to see which operating system it's on, then selects an appropriate subclass. On Windows, that's File::Spec::Win32, and in that module it defines the directory separator.

You want it to pick something else. You could add a subclass for UNC so the module handles the root directory and other things correctly, or you can make File::Spec think that it's on another system. I sometimes need to do this to work with paths from other systems without turning them into UNIX paths.

#!/usr/bin/perl

BEGIN {
    local $^O = 'MSWin32'; # or Unix MacOS VMS, and so on
    require File::Spec::Functions;
    File::Spec::Functions->import;
    }
    
my $path = catfile( 'top', 'middle', 'bottom' );

print $path, "\n";
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I run this on FreeBSD and get "top\middle\bottom".