Win8 Strawberry 5.8.9.5 (32)  Mon 09/06/2021  1:53:18
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
use Data::Dump qw(dd);  # for debug

my @words = (  # added a few extra 'words'
'', 'x', 'a', 's', 'aa', 'as', 'es', 'is', 'os', 'sh', 'si', 'so'
);
dd '@words:', \@words;  # for debug

# map words to sorted words.
my %sorted = map { $_ => join('', sort split //) } @words;
dd 'sorted words hash:', \%sorted;  # for debug

print "----------------------------\n";

for my $regex (qw(.?a?  a?.?  s?.?  .?s?)) {

    my $rx_full = qr{ ^ $regex $ }x;      # faster if used often
  # my $rx_full = qr{ \A $regex \z }xms;  # i prefer \A \z \Z and /xms tail

    print "Matches for FULL regex pattern: $rx_full \n";

    for my $word (@words) {
        print "'$sorted{$word}' -> '$word'  " if $sorted{$word} =~ $rx_full;
        }

    print "\n----------------------------\n";

    }

^Z
(
  "\@words:",
  ["", "x", "a", "s", "aa", "as", "es", "is", "os", "sh", "si", "so"],
)
(
  "sorted words hash:",
  {
    ""   => "",
    "a"  => "a",
    "aa" => "aa",
    "as" => "as",
    "es" => "es",
    "is" => "is",
    "os" => "os",
    "s"  => "s",
    "sh" => "hs",
    "si" => "is",
    "so" => "os",
    "x"  => "x",
  },
)
----------------------------
Matches for FULL regex pattern: (?x-ism: ^ .?a? $ )
'' -> ''  'x' -> 'x'  'a' -> 'a'  's' -> 's'  'aa' -> 'aa'
----------------------------
Matches for FULL regex pattern: (?x-ism: ^ a?.? $ )
'' -> ''  'x' -> 'x'  'a' -> 'a'  's' -> 's'  'aa' -> 'aa'  'as' -> 'as'
----------------------------
Matches for FULL regex pattern: (?x-ism: ^ s?.? $ )
'' -> ''  'x' -> 'x'  'a' -> 'a'  's' -> 's'
----------------------------
Matches for FULL regex pattern: (?x-ism: ^ .?s? $ )
'' -> ''  'x' -> 'x'  'a' -> 'a'  's' -> 's'  'as' -> 'as'  'es' -> 'es'  'is' -> 'is'  'os' -> 'os'  'hs' -> 'sh'  'is' -> 'si'  'os' -> 'so'
----------------------------