#!/usr/bin/perl -slw

sub mysubstr : lvalue 
{
  print "mysubstr(@_)";
  print "Returning value '",substr($_[0],$_[1],$_[2]),"' as lvalue.";
  substr($_[0],$_[1],$_[2]);
}

sub swab{
  my $s = "swab$ARGV[0]";
  goto &$s;
}

sub swab1{
    substr( $_[0], $_[1], 1 ) ^=
    substr( $_[0], $_[2], 1 ) ^=
    substr( $_[0], $_[1], 1 ) ^=
    substr( $_[0], $_[2], 1 );
}

sub swab2{
    mysubstr( $_[0], $_[1], 1, "1" ) ^=
    mysubstr( $_[0], $_[2], 1, "2" ) ^=
    mysubstr( $_[0], $_[1], 1, "3" ) ^=
    mysubstr( $_[0], $_[2], 1, "4" );
}

sub swab3{
  my($c1,$c2);
  print "   Char $_[2]: ",$c2 = substr($_[0],$_[2],1);
  print "   Char $_[1]: ",$c1 = substr($_[0],$_[1],1);
  print "        1 ^ 2: ",$c1 = $c1 ^ $c2;
  print "    2 ^ 1 ^ 2: ",$c2 = $c2 ^ $c1;
  print "1 ^ 2 ^ 2 ^ 2: ",$c1 = $c1 ^ $c2;
  substr($_[0],$_[1],1)=$c1;
  substr($_[0],$_[2],1)=$c2;
}

my $t;

$t ='AB';
swab( $t, 0, 1 );
print "'$t'";
print "'$t'";
print "length: ",length($t);

$t ='AB';
swab( $t, 0, 1 );
print "'$t'";
print "'$t'";
print "length: ",length($t);