For a different variation on the same theme, try

$string =~ s/(?<!\\)'/\\'/g;
[download]

Though this raises the question "what if the string contains a backslash followed by a single quote?" Which makes the regex slightly more complicated, and leads back to he conclusion that quotemeta might not be such a bad idea, after all...

Update: while I'm writing silly regexes...

$string =~ s/(?<!\\)(?=')/\\/g;
[download]

<duck>