$ perl -le'
my $string = "abcdefghi";
print $string;
# get the address of every character
my @addr = map \substr( $string, $_, 1 ), 0 .. length( $string ) - 1;
# swap the characters in-place.
( ${$addr[ $_ ]}, ${$addr[ -( $_ + 1 ) ]} ) = ( ${$addr[ -( $_ + 1 ) ]}, ${$addr[ $_ ]} ) for 0 .. $#addr / 2;
print $string;
'
abcdefghi
ihgfedcba