Metadoktor's solution certainly will work, but I would like to offer one of my own.

#! /usr/bin/perl -Tw

my ($label, $string);
$string = "a1:  a,b,c";
$string =~ s/\s+//g;  # $string = a1:a,b,c
($label, $string) = split(/:/,$string);
my @tokens = split(/,/,$string);
foreach (@tokens)
{
  print "$_\n";
}
[download]

This should print: a b c

However, you MIGHT also be asking exactly how to eliminate leading spaces. The '^' at the beginning of the regex points to the beginning of a line or string such that:

$string =~ s/^\s*//;

will get rid of leading spaces