I have gotten into the habit of checking options using defined just in case 0 is a legal value. The code you show treats 0 as an undefined value.

$ ./test.pl -u 0
$opt_u=Undefined
$opt_h=Undefined
$opt_p=Undefined
$opt_k=Undefined
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I realize that in this particular application, 0 is unlikely to be a username, password, etc., but it does not hurt to account for this corner case:

print "\$opt_u=";
print defined $opt_u ? $opt_u : "Undefined" ;
print "\n";
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