Assuming those two spans of characters are all you're interested in (rather than the entire line as a series of :-delimited fields), then it sounds to me like you're looking for a regex-based solution, such as:

$LINE =~ /^([^:]*):[^:]:[^:]:[^:]:([^,:]*)/;
$up_to_first_colon = $1;
$fourth_colon_to_next_comma = $2;
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The ^([^:]*): captures all of the non-colon characters ([^:]*) from the start of the line (^) until the first colon and places it in $1.

Each [^:]: skips over a group of non-colon characters, ending with a colon.

Finally, ([^,:]*) captures the group of non-colon, non-comma characters following the fourth colon and places it in $2. (Change it to ([^,]*) if the second field should only be terminated by a comma and not by a colon.)