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sub fibonacci {
    my $n = shift;
    return 1 if $n == 1 or $n == 0;
    return fibonacci($n-1) + fibonacci($n-2);
}

sub fibonacci {
    die "..." unless defined $_[0];
...
    die "..." if $n != int $n;
    # ...
}

sub fibonacci (PositiveInt $n) {
    return 1 if $n <= 1;
    return fibonacci($n-1) + fibonacci($n-2);
}