#! perl -slw
use strict;

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

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

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

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

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

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

__END__
P:\test>junk
'BA'
' A'
Subroutine swab redefined at (eval 1) line 2.
'BA'
' B'

##</code><code>##

#! 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'
'  '