my $s = "a1: a,b,c";

# instead of just a colon, first split on colon preceded
# or followed by optional whitespace
my ($prefix, $rest) = split /\s*:\s*/, $s;

# now, $prefix = "a1" and $rest = "a,b,c"

# now do the same for commas
my @rest = split /\s*,\s*/, $rest;

# now, @rest = ( 'a', 'b', 'c' )