The g modifier does not tell the regex engine to start matching again from the beginning, but to continue matching from where you've arrived.

The following regex:

$foo =~ s/([:,{])(.)/$1 $2/g;
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is matching two characters, i.e. :{, so that the regex engine will continue on the next character, i.e. ", which cannot be matched by the regex. So, in short, your regex is not applied a second time on your input string. It has gotten too far already in your input string.

If you want the regex engine to repeatedly match each of the input characters, please remove the (.) part:

$foo =~ s/([:,{])/$1 /g;
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For example (demonstrated under the Perl debugger):

  DB<1> $foo = ':{"';

  DB<2> $foo =~ s/([:,{])/$1 /g;

  DB<3> print $foo
: { "
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Update:: this is essentially the same solution as toolic's proposal, I did not notice until I reviewed the thread after having posted my response.