I simply meant that I don't see any easy way of avoiding the goto in these mutually recursive routines:

#! perl -slw
use strict;

my( $count1, $count2 );

sub a1 {
    b1( $_ [ 1 ] );
    goto &a2;
}

sub a2 {
    $count1++;
    $_[ 0 ]-- and goto &b2;
}


sub b1 {
    $_[ 0 ]-- and goto &b2;
}

sub b2 {
    $count2++;
    goto &a2;
}

my( $n1, $n2 ) = ( 100, 50 );
a1( $n1, $n2 );

print "c1: $count1; c2: $count2";

__END__
[15:37:55.16] P:\test>mutrec
c1: 151; c2: 150
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This is not a particularly useful example, but imagine that one (pair) are reading a huge XML file for instance, and the other is validating it.

Rather than having to read the whole file, the first reads a bit and the second processes a bit and the first reads a bit more and so on. Both retain their own state across calls to the other. Think coroutines; a consumer and producer that each retain their running state, whilst switching control between themselves