>perl -wMstrict -le
"for my $str (@ARGV) {
   printf qq{string '$str': };
   my ($first, $last) = $str =~ m{ \A . | . \z | \z }xmsg;
   $last = $first if not $last;
   print qq{first '$first', last '$last'};
   }
"     ""  "a"  "ab"  "abc"  "abcd"
string '': first '', last ''
string 'a': first 'a', last 'a'
string 'ab': first 'a', last 'b'
string 'abc': first 'a', last 'c'
string 'abcd': first 'a', last 'd'