This should also work:

#example 1
$content =~ s/(.{1,$width}) /$1<BR>/g;
print $content;
[download]

or

#example 2
$content =~ s/(.{1,$width}) |([^ ]{$width})/$1$2<BR>/g;
print $content;
[download]

The $width represents the largest number of characters each line should have.
The first example does not break single words longer than the variable $width specifies. The second will.