sub recv_data{
    if(read_some_data){
        return($stuff1, $stuff2);
    } else {
        return;
    }
}

##</code><code>##

>perl -le
"use warnings FATAL => 'all';
 use strict;
 ;
 my @pairs1 = qw(1 2 3 4);
 ;
 sub S1 {
   return if not @pairs1;
   return pop @pairs1, pop @pairs1;
   }
 ;
 my ($x, $y);
 while (($x, $y) = S1()) {
   print qq{$x $y};
   }
 ;
 my @pairs2 = qw(5 6 7 8);
 ;
 sub S2 {
   return undef if not @pairs2;
   return pop @pairs2, pop @pairs2;
   }
 ;
 while (($x, $y) = S2()) {
   print qq{$x $y};
   }
"
4 3
2 1
8 7
6 5
Use of uninitialized value $x in concatenation (.) or string at ...