sub repeating {
    my( $string ) = @_;
    my( $pattern, $repeat, $end ) = $string =~ /^(.+?)(\1+)(.*)$/;
    while( defined $pattern ) {
        return $pattern
            if  length($end) <= length($pattern)
            &&  $end eq substr($pattern,0,length($end));
        print "($pattern) wasn't long enough.\n";
        ( $pattern, $repeat, $end ) =
            $string =~ /^(\Q$pattern$repeat\E.+?)(\1+)(.*)$/
    }
    return undef;
}
my $pattern = repeating( "aabaabaabcaabaabaabca" );
printf "(%s) wins\n", $pattern
    if  $pattern;
__END__
(a) wasn't long enough.
(aab) wasn't long enough.
(aabaabaabc) wins