Unless you understand why the warning is being generated and are sure that undef is a valid value you should not turn the warning off. Even if you do decide to turn off the warning, you should constrain the scope of no warnings to the smallest scope possible:

    {
    no warnings 'uninitialized';
    $j =~ s/\;/\,/gi;
    }
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However it's not at all clear from your code that turning off the warning is appropriate. Make sure you understand what's going on before you opt for that solution!