Untested:

my $SHARES = 1000;
my $DIVIDEND = 0.0678;
my $YEARS = 20;
sub stock {
    my ($shares, $years) = @_;
    $years >= 1 ? stock(int($shares * (1 + $DIVIDEND)), $years - 1) : 
+$shares;
}
print stock $SHARES, $YEARS;
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