What do you mean by brute forcing this?

$pre = "PRE";
@S = qw/Q PREQ/;
s/^(?!$pre)/$pre/ for @S;
say for @S;
[download]

or

$pre = "PRE";
@S = qw/Q PREQ/;
for (@S) {
    $_ = $pre . $_ unless 0 == index $_, $pre;
}
say for @S;
[download]