Thanks for all your replies.
That's very thorough little test harness there Browser, nice. I think your going to beat me when you see that the nub of the question was just all about return values. This finally works

#!/usr/bin/perl


dofistcritical() or dobackupcritical() or dofailed();


sub dofistcritical {if (int rand 5)
    {print "hooray it worked first time!\n"}
    else {return 0}}

sub dobackupcritical {if (int rand 5)
    {print "First time failed but luckily the backup worked...phew\n"}
    else {return 0}}

sub dofailed {if (int rand 5)
    {print "Main failed. Backup Failed. You have 3 minutes to reach a 
+safe distance\n"}
    else {return 0}}
[download]

I'm not exactly sure where it was failing before, but the complicated blocks to force critical and non-critical areas must have had me seeing things, from here I can work the critical/non-critical sections back in.
Cheers all.