use v5.14;
use Data::Dump qw/pp/;

my @chars = ('0'..'9', 'a'..'z', 'A'..'Z');
# Up to scalar(@chars) different words (actually @chars+1 because of undef, but that wouldn't help readability)

$_ = <<STR;
Jack and Jill went up the hill to fetch a pail of water
Jack fell down and broke his crown and Jill came tumbling after
STR

my @words = /\w+/g;
my %replace;
my $asChars = join '', map { $replace{$_}//=shift(@chars) } @words; # 'defined-orcish manoeuver' :D
# say pp \%replace;
say $asChars;
my %reverse = reverse %replace;
say join ' ', map $reverse{$_}, split //, $asChars;


__DATA__
0123456789abc0de1fgh12ijk
Jack and Jill went up the hill to fetch a pail of water Jack fell down and broke his crown and Jill came tumbling after