How do I break an outer loop from an inner one in Perl?

Use a label:

OUTER:
foreach my $x (@x) {
 foreach my $y (@z) {
  foreach my $z (@z) {
   if (something()) {
    last OUTER;
   }
   # do stuff 
  }
  # do stuff
 }
 # do stuff
}

The "last LABEL" syntax is described in the documentation.