No sooner asked than solved. Two words, two colons: Text::Reform. The above can be implemented as:

use Text::Reform;

my $TABLE = << '__FORMAT__';
<<<<<<<<<<<< <<<<<<<<<<<<<<<<< [[[[[[[[[[[[[[[[[[[[[[[[[
__FORMAT__

sub continue_line {
    my ($text, $reqlen, $fldlen) = @_;
    if ($reqlen == $fldlen) { $text =~ m/\A(\s*\S*)(.*)/s }
    else                    { (" "x$reqlen . "\\", $text) }
}

print form
  { break => \&continue_line },
  $TABLE, $class, $name, $args;

An *extremely* useful module!