here new code to highlight whats happening.

Please note the recursion here is effectively used to implement 4 nested loops, see second part of the code

#https://perlmonks.org/?node_id=11124854

use strict;
use warnings;
use Data::Dump qw/pp dd/;
use 5.10.0;


sub permutation {
    my ($perm,@set) = @_;

    my $level  = 5 - @set;
    my $marker = " "x$level . "$level: " ;
    say "$marker ENTER perm='$perm' set=(@set)";

    unless (@set) {
        say "$marker RESULT: $perm";
    } else {
        permutation( $perm.$set[$_],
                     @set[0..$_-1], @set[$_+1..$#set]
                   ) for (0..$#set);
    }

    say "$marker RETURN";
    return;
}

my @input = (qw/a b c d/);

permutation('',@input);


my @set = @input;
my $perm = "";

for (0..3) {                            # level 1
    my @set =@set;
    my $perm = $perm . splice @set,$_,1;
    for (0..2) {                        # level 2
        my @set =@set;
        my $perm = $perm . splice @set,$_,1;
        for (0..1) {                    # level 3
            my @set =@set;
            my $perm = $perm . splice @set,$_,1;
            for (0..0) {                # level 4
                my @set =@set;
                my $perm = $perm . splice @set,$_,1;

                say $perm;              # level 5
            }
        }
    }
}
[download]

(shortened) output

-*- mode: compilation; default-directory: "d:/tmp/pm/" -*-
Compilation started at Wed Dec  9 13:22:51

C:/Perl_524/bin\perl.exe -w d:/tmp/pm/permute.pl 
 1:  ENTER perm='' set=(a b c d)
  2:  ENTER perm='a' set=(b c d)
   3:  ENTER perm='ab' set=(c d)
    4:  ENTER perm='abc' set=(d)
     5:  ENTER perm='abcd' set=()
     5:  RESULT: abcd
     5:  RETURN
    4:  RETURN
    4:  ENTER perm='abd' set=(c)
     5:  ENTER perm='abdc' set=()
     5:  RESULT: abdc
     5:  RETURN
    4:  RETURN
   3:  RETURN
   3:  ENTER perm='ac' set=(b d)
    4:  ENTER perm='acb' set=(d)
     5:  ENTER perm='acbd' set=()
     5:  RESULT: acbd
     5:  RETURN
    4:  RETURN
    4:  ENTER perm='acd' set=(b)
     5:  ENTER perm='acdb' set=()
     5:  RESULT: acdb
     5:  RETURN
    4:  RETURN
   3:  RETURN
### shortened ...

### Nested Loops
abcd
abdc
acbd
acdb
adbc
adcb
bacd
badc
bcad
bcda
bdac
bdca
cabd
cadb
cbad
cbda
cdab
cdba
dabc
dacb
dbac
dbca
dcab
dcba

Compilation finished at Wed Dec  9 13:22:51
[download]