But no while-loop at all is needed (and no captures) if lookarounds are used. (I'm substituting  '.' rather than a space in the examples below for greater clarity — I hope.)

Inserting something after each character in the class including when it's at the end of the string:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @foos = (':X,{X', ':X,{');
 ;;
 for my $foo (@foos) {
   printf qq{'$foo' -> };
   $foo =~ s( (?<= [:,{]) ){.}xmsg;
   print qq{'$foo'};
   }
"
':X,{X' -> ':.X,.{.X'
':X,{' -> ':.X,.{.'
[download]

Inserting something after each character in the class except when it's at the end of the string:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @foos = (':X,{X', ':X,{');
 ;;
 for my $foo (@foos) {
   printf qq{'$foo' -> };
   $foo =~ s( (?<= [:,{]) (?= .) ){.}xmsg;
   print qq{'$foo'};
   }
"
':X,{X' -> ':.X,.{.X'
':X,{' -> ':.X,.{'
[download]

(These examples work with Perl 5.8. Note that with the Perl 5.10+ regex  \K operator, the  (?<= [:,{]) terms in both examples can simplify to  [:,{] \K instead.)

Update: Note also that in all cases  (?! \z) can perhaps better express the "except at the end of the string" requirement than  (?= .) — and may even be slightly faster!