Setting default values in getopts

neilwatson has asked for the wisdom of the Perl Monks concerning the following question:

Givers of wisdom and knowledge please consider my humble questions:

How does one set default values of command line arguements?

use Getopt::Std;

my %opt;
my $opt{d} = "x";
my $opt{a} = "0";
my $opt{s} = "0";
my $opt{e} = "x";
getopts("dase:", \%opt);
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Using something like this returns a syntax error at my $opt{d}="";. What have I done wrong?