If you look at perldoc File::Copy you will see the following:
RETURN All functions return 1 on success, 0 on failure. $! will be set if an error was encountered.Therefore, your code should be exposing what is in $! if there is an error:
if ( copy($source_file, $target_file)) { print "success\n";} else { warn "fail: $!\n";}Also, as described in the documentation for File::Copy, copy returns 1 on success (a true value), so I removed your == 0 in the success test. With Perl, any true value in the if(COND){...} statement will do; you don't need to explicitly test for 1.
Regarding paths: The / character can be used as a path delimiter, even if you're using Windows except in some cases where you might be sending the path to an external program. This capability allows you to relatively portably write code that expresses paths as foo/bar/baz, and it will work with Windows similarly to how it would work under a *nix operating system. Using the forward slash as a delimiter allows you to avoid escaping every backslash in a path: foo\\bar\\baz.