The /g modifier prevents you from matching overlapping pieces of texts, so don't do that.

A workaround is to only match the first word normally, and match the second one in a look-ahead:

my $text = "hello to all the perl monks";
while ($text =~ /\b([A-Za-z'\-]+) (?=([A-Za-z'\-]+))\b/g) {
    print "$1 $2\n";
}
[download]

(Gives the desired output).

The key is that look-ahead groups (?=...) match, but don't consume any characters, so the position of the next match is not affected by what that group matched. See perlre for details, or "Mastering Regular Expression" by J. Friedl.