#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';
use Math::Combinatorics;
use List::MoreUtils qw(indexes);
use Data::Printer;

my $line = "L1 H1 L2 H2";
my @input = split /\s/, $line;

# segregate classes
my @h_vals;
my @l_vals;
for (@input) {
    push @h_vals, $_ =~ /^H\d+/g;
    push @l_vals, $_ =~ /^L\d+/g;
}

# index input based on classes
my @h_indexes = indexes { $_ =~ '^H\d+' } @input;
my @l_indexes = indexes { $_ =~ '^L\d+' } @input;

# permute values within classes
my @h_perms = permute @h_vals;
my @l_perms = permute @l_vals;

# get all permutations of recombined classes
my @perms;
for my $h_cur (@h_perms) {
    for my $l_cur (@l_perms) {
        my @current;
        my @h = @{$h_cur};
        my @l = @{$l_cur};
        for (@h_indexes) {
            $current[$_] = shift @h;
        }
        for (@l_indexes) {
            $current[$_] = shift @l;
        }
        push @perms, \@current;
    }
}

p @perms;

__END__
[
    [0] [
        [0] "L1",
        [1] "H2",
        [2] "L2",
        [3] "H1"
    ],
    [1] [
        [0] "L2",
        [1] "H2",
        [2] "L1",
        [3] "H1"
    ],
    [2] [
        [0] "L1",
        [1] "H1",
        [2] "L2",
        [3] "H2"
    ],
    [3] [
        [0] "L2",
        [1] "H1",
        [2] "L1",
        [3] "H2"
    ]
]