Or you could use Time::Local to turn a date spec into an epoch time (as returned by time), then give that to localtime--from the docs:

    $time = timelocal($sec,$min,$hours,$mday,$mon,$year);
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So, for your case:

    use Time::Local;
    my $time = timelocal(0, 0, 0, 21, 4, 1981);
    my $wday = (localtime $time)[6];
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