How to set cronjob on wake up from sleep?

For example if you want a cron job to run after each reboot, you add something like this to your cron file:

@reboot ./do_sth

Is there something similar to that for waking up from a sleep state?

This is not something that can be managed by cron, but it can be managed by the Power Management Utilities (pm-utils). When reading man pm-action, you find:

/etc/pm/sleep.d, /usr/lib/pm-utils/sleep.d: Programs in these directories (called hooks) are combined and executed in C sort order before suspend and hibernate with as argument suspend or hibernate. Afterwards, they are called in reverse order with argument resume and thaw respectively. If both directories contain a similar named file, the one in /etc/pm/sleep.d will get preference. It is possible to disable a hook in the distribution directory by putting a non-executable file in /etc/pm/sleep.d, or by adding it to the HOOK_BLACKLIST configuration variable.

So all you need to do is create a script in /etc/pm/sleep.d that looks like this:

#!/usr/bin/env bash
action="$1"

case "$action" in
   suspend)
        # List programs to run before, the system suspends
        # to ram; some folks call this "sleep"
   ;;
   resume)
        # List of programs to when the systems "resumes"
        # after being suspended
   ;;
   hibernate)
        # List of programs to run before the system hibernates
        # to disk; includes power-off, looks like shutdown
   ;;
   thaw)
        # List of programs to run when the system wakes
        # up from hibernation
   ;;
esac

Obviously, you can alter this if you do not want to distinguish between suspend and hibernate, or resume and thaw into something like:

#!/usr/bin/env bash
action="$1"
case "$action" in
   suspend|hibernate) stuff ;;
   resume|thaw)       stuff ;;
esac