pseudocode example:

# this is the (infinite) main loop, which takes a line of
#    keyboard input and processes it, until the program is
#    killed 
while (<>) {
     # call subs here 
     if (foo) { &a(); next; }
     elsif (bar) { &b(); next; }
     else { &c; }
} 
sub a {
     # do stuff
     if (divide by zero error) &myerror(message);
     &c();
     # do more stuff
     return value;
}
sub b {
     # do a lot of stuff
     if (format is wrong) &myerror(message);
     # do more stuff
     return hash;
}
sub c {
     # do stuff
     if (type mismatch) &myerror(message);
     &b();
     return list;
}
sub myerror {
     # print error message

     # and now I want to go straight back to the main
     #      loop, no matter how many levels of subroutines
     #      deep this &myerror call was, as if the program 
     #      were just starting (with the single exception
     #      that I want to keep all the program's  
     #      global variables intact) 
}