Oh! I didn't think of that one. But I'm not sure why you have to do all those extra verifications. Since the input data is a repeated pattern, the length of that pattern is how little you can shift your string to the left and have a perfect match.

$s = 'abcdabcdabceabcdabcdabceab';
for (1..length($s)-1)
{
        print substr($s,0,$_) and last if substr($s,$_) eq substr($s,0
+,length($s)-$_);
}
[download]

abcdabcdabce