my @chunks = ($string =~ /(?|(?:((?:[^\n]*\n){19}[^\n]*)\n)|((?s:.*)))/g);
print Dumper @chunks[0];

##</code><code>##

 Sequence (?|...) not recognized in regex; marked by <-- HERE in m/(?| <-- HERE (?:((?:[^\n]*\n){19}[^\n]*)\n)|((?s:.*)))/.