Win8 Strawberry 5.30.3.1 (64)  Sat 12/19/2020 13:18:14
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
use 5.014;  # for s/// with /r modifier

print "Content-type: text/plain\n\n";

print "Test\n\n";

# sub undent {  # works, but not as flexible
#     return $_[0] =~ s/^\s+//gmr;
#     }

sub undent {
    my $text = shift;
    $text =~ s{ \A [^\n]* \n (\s*) }{}xms;
    my $dent = length $1;
    return $text =~ s{ ^ \s{0,$dent} }{}xmsgr;
    }

if (1) {
    print undent "
    Here is
    some test text
        that can
  also be
          ragged
    with plenty of space
    at the start\n"
    }
^Z
Content-type: text/plain

Test

Here is
some test text
    that can
also be
      ragged
with plenty of space
at the start
[download]

However, this has the disadvantage that you have to manually stick a newline at the end to get a nice block ending. But I suppose you could tack that on automatically with a .= "\n" or some such.
(Update: I realized after posting that I only tested with spaces as indentation characters, not tabs, which I never use anyway. I think the code would work with pure tab indents, but I haven't tested this. I doubt the code would work with a mixture of spaces and tabs used as indentation.)

Update: Yes,
    return $text =~ s{ ^ \s{0,$dent} }{}xmsgr . "\n";
seems to do it for the automatic newline text block end.