# $s does not match, but '12' would be consumed in the 
# match attempt.  In this case, all of $s is consumed
# so $s should be able to match if additional characters
# were added
$s = '12'
$r = qr/^123$/;
could_match( $r, $s )  # returns TRUE

# $s does not match and none of $s would be consumed in the
# attempt as the '1' in the regex could not match.  Since
# $s was not consumed in it's entirety in the match attmempt
# there are no characters that could be added to $s to allow
# it to match $r
$s = '23'
$r = qr/^123$/;
could_match( $r, $s )  # returns FALSE

# $r in this case is not anchored, so it could always match
# if more characters were added.  However, since none of $s
# was used in the match attempt, the entirety of $s could
# not match even if more characters were added.
$s = '123';
$r = qr/456/;
could_match( $r, $s )  # returns FALSE