# either this way:
$s =~ s/^\X{0,$MAX_CHARS}\K.*//s;

# or "by hand", this way:
substr($s, pos $s) = "" if $s =~ /^\X{0,$MAX_CHARS}/g;

##</code><code>##

3768         case CLUMP: /* Match \X: logical Unicode character.  This is defined as
3769                        a Unicode extended Grapheme Cluster */
3770             /* From http://www.unicode.org/reports/tr29 (5.2 version).  An
3771               extended Grapheme Cluster is:
3772 
3773                CR LF
3774                | Prepend* Begin Extend*
3775                | .
3776 
3777                Begin is (Hangul-syllable | ! Control)
3778                Extend is (Grapheme_Extend | Spacing_Mark)
3779                Control is [ GCB_Control CR LF ]
3780 
3781                The discussion below shows how the code for CLUMP is derived
3782                from this regex.  Note that most of these concepts are from
3783                property values of the Grapheme Cluster Boundary (GCB) property.