#!/usr/bin/perl -w 
my $username = 'joejoe'; 
my $password = 'lightning'; 
my $name = 'Joe';
my $SQL = "SELECT FROM table WHERE" .
          (($username)?" username = '$username'":'') .
          (($password)?" and password = '$password'":'') .
          (($name)?" and name = '$name'":'') . ';';
print $SQL;