#! perl -slw
use strict;

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

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

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

__END__
P:\test>swab
'BA'
'BA'
' A'
' A'