#! perl -slw
use strict;

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

sub swab2{
	substr( $_[0], $_[1], 1 ) ^=
	substr( $_[0], $_[2], 1 ) ^=
	substr( $_[0], $_[1], 1 ) ^=
	substr( $_[0], $_[2], 1 )
}

my $s ='AB';
swab1( $s, 0, 1 );
print "'$s'";

swab2( $s, 0, 1 );
print "'$s'";

my $u = 'AB';
swab1( $u, 0, 1 );
print "'$u'";

swab2( $u, 0, 1 );
print "'$u'";

__END__
P:\test>junk
'BA'
'AB'
' A'
'  '