#! perl -slw
use strict;

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

my $s = 'AB';
swab( $s, 0, 1 ), print "'$s'" for 1 .. 10;
__END__
P:\test>test
'BA'
'AB'
'BA'
'AB'
'BA'
'AB'
'BA'
'AB'
'BA'
'AB'