G'day Rolf,

That's a valid point. My main intent with that code was really to show the complexity of the solution when a regex or module were not used. Anyway, adding a little more complexity, you can trim whatever blanks you want:

$ perl -E '
    my @blanks = (" ", "\n", "\r", "\t");
    my @x = (
        "  a b c ", "d  e f \r ", " \t g h  i",
        "j k l", " ", "\n", "\n\nXYZ\n\n", ""
    );

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

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

        while (grep { 0 == index $str, $_ } @blanks) {
            $str = substr $str, 1;
        }

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

        while (grep { $str_end == rindex $str, $_ } @blanks) {
            $str = substr $str, 0, $str_end;
            --$str_end;
        }

        $x[$i] = $str;
    }

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

XYZ

|
||
*** Final strings ***
|a b c|
|d  e f|
|g h  i|
|j k l|
||
||
|XYZ|
||
[download]

You're quite correct about "The OP should be clearer ...". The word 'blank' is often used to mean various things: a single space, multiple consecutive spaces, a whitepace character, multiple consecutive whitepace characters, and I have also seen it used to refer to a zero-length string. Similarly, the word 'space' can mean a single space, any gap between visible characters, and so on. So, as with many posts, we're left with guessing the most likely meaning from the context.

My belief, that a regex is a better option, strengthens as the complexity of the non-regex and non-module code increases. :-)