See use and require.

Basically, use is compile-time and will happen before you assign any variables, so use $OsDep will not work. I would rewrite the code to:

if (...) {
    require Win32::EventLog;
    Win32::EventLog->import();
} elsif (...) {
    require File::Copy;
    File::Copy->import();
} else {
    warn "Unknown OS $^O detected, using default module(s).";
    require File::Copy;
    File::Copy->import();
};
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