#! /usr/bin/env perl
use 5.014; use warnings;

# We're going to try matching substrings of this...
my $MASTER_STR = 'abcdabcdabceabcdabcdabceab';

# Using this pattern to find (possibly truncated) repetitions...
my $REPETITION_PAT = qr{

    \A         # Start at the beginning of the string
    (.+?)      # Match minimal initial sequence (as $1)
    (?{$^N})   # Remember it internally (as $^R)

    (?|        # Then either it's not truncated...
        \1+        # Rematch it exactly as often as possible
        \z         # ...until the end of the string
        ()         # Remember that nothing was left over (as $2)

    |          # Or else it is truncated...
        \1*        # Rematch it exactly as often as possible (or not)
        (.+)       # Then grab what's left (as $2)(internally as $^N)
        \z         # ...until the end of the string

        # Then verify that what's left is a proper truncation...
        (??{
            # If what's left ($^N) is substring of repetition ($^R)...
            $^N eq substr($^R,0,length($^N))
                ? q{}       # ...then do nothing (i.e. keep matching)
                : q{(?!)}   # ...else initiate backtracking
        })
    )
}xms;

# Loop through increasingly truncated substrings...
for my $substr (1..length($MASTER_STR)) {

    # Compute the substring...
    my $str = substr($MASTER_STR,0,-$substr);

    # Evaluate the match...
    if ($str =~ $REPETITION_PAT) {
        say "$str\n     Matched: $1*$2";
    }
    else {
        say "$str\n     Didn't match";
    }
}