One approach is to use die to send the exception (with a message) and blockwise eval to catch it. Example:

{
  package foo;
  sub add {
    my ($x, $y) = @_;
    die "not a number\n" unless $x =~ /\d/ and $y =~ /\d/;
    $x + $y
  }
}

# catch any exceptions
eval {
  print "1+2==", foo::add(1, 2);
  print "a+2==", foo::add("a", 2);
};

# handle any exceptions caught by eval
if ($@) {
  push @errors, $@;
}

# program continues...
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Note that the subroutine being in a module doesn't really change things much. Your question is about returning out-of-band data. Using a global variable is one approach, and here I've shown another. These techiques work the same whether the subroutine is in the same package or not.