There are a lot of good suggestions above this, however if you want to know why this specific apprach to solving this problem doesn't work, the problem is in this line:

elsif ($last =~ m/([A-Za-z\'\-]+)/) {
        $surname = $1;
}
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If you're looking at the name "Joe De Bloggs" then $last, by this point equals "De Bloggs". Once you do this match (and it will match as you do have one or more of those characters in $last), $surname gets set to "De".

If you swapped the two conditions so that you had:

elsif($last =~ m/(\w+)\s+(\w+)/) {  # note the + here.
     $surname = join(' ', $1, $2);  # note change to $2
}
elsif($last =~ /([A-Za-z\'\-]+)/) {
        $surname = $1;
}
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Then $surname will end up with "De Bloggs" as you desire.

Good luck.