Cleaned up and annotated. It's a perl version of https://en.m.wikipedia.org/wiki/Permutation#Generation_in_lexicographic_order

#!/usr/bin/perl -l
use strict; use warnings;
$_ = 321; # for testing, change to 987654321 for final run

1 while print,
  s/.*\K # find the last
  (.) # digit such that
  (.*) # there is a later (latest)
  (.)(??{$1 < $3 and 'x'}) # digit less than it
  (.*) # and get rest
  # swap those two digits ( $1 & $3 )
  # then reverse everything after the first swapped digit
  / $3 . reverse $2.$1.$4 /xe
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