You might not have noticed, but the print command can take a list of things to print, which means that join() is really not required. The syntax error is that after your first string where you establish the font, you don't use the period to concatenate, or a comma to make a proper list.

foreach (0..2)
{
     print '<font face="courier"><p>',
             substr($strings[$_], $x, $line_length),
             '</p></font>',
             $/;
}
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I moved the for-loop into a more obvious location. Before you were losing it in the print statement. I've also take then liberty of opening your <FONT> tag properly, as well as using single quotes to avoid having to escape the quotation marks in your HTML.

As another note, print sends to STDOUT by default, so there's no real need to specify that.