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>perl -wMstrict -le
"use feature 'state';
...
'2,3,5,6,7,9,11,12,13,14,16,17,19'

'2,3,5-7,9,11-14,16,17,19'

$s =~ s{
  (?<! \d) ($n)   (?{   order($^N) })
  (?: $sep ($n) (?(?{ ! order($^N) }) (*F))){2,}
  }
  {$1-$2}xmsg;

sub order {
  state $p = 0;
...
  (?: $sep ($n)  (?(?{ ! order() }) (*F))){2,}
  }
  {-$^N}xmsg;