This seems to pass my limited testing...

use strict;
use warnings;

my $string = '1,2,a,b,3';

$string =~ s/( (?:(?<!\d)(?:,|\.)(?!\s*\d))     |
               (?:(?<=\d)(?:,|\.)(?=\s*)(?!\d)) |
               (?:(?<!\d)(?:,|\.)(?=\s*\d))     )
            /$1 /gx;

print $string, "\n";
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It's really ugly, but triggers a substitution all of the following cases:

• Comma or dot is not surrounded by digits, with optional whitespace after the comma or dot.
• Comma or dot doesn't have a digit to the left, with optional whitespace after the comma or dot.
• Comma or dot doesn't have a digit to the right, with optional whitespace after the comma or dot.

So I think it meets your spec. If this is homework I would seriously recommend immersing yourself in the POD's so that you'll be able to explain to your professor how it is you came up with such a wild RE. For that matter, it's time that I re-read the POD's, as I'm not convinced that it really needs to be so explicit to work properly.

See perlre to learn about zero-width lookahead and lookbehind (positive and negative). Also read perlretut and perlop under the "Regexp quote like characters" section, for starters.