using Text::Levenshtein's distance function, I get source1 as the best match.

use Text::Levenshtein qw(distance);

$target = 'source';
$best=undef;
for (qw(donor_dedupe_source indicted_video_source source1)) {
    $dist = distance($target,$_);
    if ($best == undef || $dist < $best) {
        $best = $dist;
        $bestword = $_;
    }
}
print "Best word was $bestword with an edit distance of $best\n";
[download]

output: Best word was source1 with an edit distance of 1

update: changed default value of $best to prevent perfect matches from being lost