my @where;
my $sql = 'select * from table';

push @where, 'column1 = "' . $var1 . '" ' if defined $var1;
push @where, 'column2 = "' . $var2 . '" ' if defined $var2;
push @where, 'column3 = "' . $var3 . '" ' if defined $var3;

$sql = $sql . ' where ' . join(' and ', @where);