Find the day of the week of the first day of the month (using Date::Calc, DateTime, localtime or whatever).

Find the number of days to add or subtract.

Days to first Monday:

$days_to_first_monday = ($dow + 7 - 1) % 7;
[download]

Days to previous Friday:

$days_to_last_friday = ($dow + 7 - 5) % 7 - 7;
[download]

In both case, $dow is expected to hold 1..7 for Mon..Sun or 0..6 for Sun..Sat

Use Date::Calc or DateTime to perform the math.