$ perl -E '
    my @x = ("  a b c ", "d  e f  ", " g h  i", "j k l", " ", "");

    say "*** Initial strings ***";
    say "|$_|" for @x;

    for my $i (0 .. $#x) {
        my $str = $x[$i];

        while (0 == index $str, " ") {
            $str = substr $str, 1;
        }

        my $str_end = length($str) - 1;

        while ($str_end == rindex $str, " ") {
            $str = substr $str, 0, $str_end;
            --$str_end;
        }

        $x[$i] = $str;
    }

    say "*** Final strings ***";
    say "|$_|" for @x;
'
*** Initial strings ***
|  a b c |
|d  e f  |
| g h  i|
|j k l|
| |
||
*** Final strings ***
|a b c|
|d  e f|
|g h  i|
|j k l|
||
||