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if(  need_module_X()  ) {
    if(  ! eval { require Module::X; 1 }  ) {
...
        Module::X->import(  qw( A B C )  );
    }
}

BEGIN {
    if(  ! eval { require Module::X; 1 }  ) {
...
        Module::X->import(  qw( A B C )  );
    }
}