use Algorithm::Loops qw( NextPermute );

my @list = sort qw( a b c );
do {
   print(join(', ', @list), "\n");
} while (NextPermute(@list));

##</code><code>##

There are 24 permutations of abcd, 4 taken at a time:
   abcd, abdc, acbd, acdb, adbc, adcb,
   bacd, badc, bcad, bcda, bdac, bdca,
   cabd, cadb, cbad, cbda, cdab, cdba,
   dabc, dacb, dbac, dbca, dcab, dcba
  
There is   1 combination  of abcd, 4 taken at a time:
   abcd

There are 24 permutations of abcd, 3 taken at a time:
   abc, acb, bac, bca, cab, cba,
   abd, adb, bad, bda, dab, dba,
   acd, adc, cad, cda, dac, dca,
   bcd, bdc, cbd, cdb, dbc, dcb
  
There are  3 combinations of abcd, 3 taken at a time:
   abc, abd, acd, bcd