P:\test>perl -l
sub swab{ substr($_[0],$_[1],1)^=substr($_[0],$_[2],1)^=substr($_[0],$_[1],1)^=substr($_[0],$_[2],1); }
$s='AB';
swab($s,0,1), print "'$s' : ", sprintf "%02x "x2, unpack 'C*',$s for 1..10;
^Z
'BA' : 42 41
' B' : 00 42
'  ' : 00 00
'  ' : 00 00
'  ' : 00 00
'  ' : 00 00
'  ' : 00 00
'  ' : 00 00
'  ' : 00 00
'  ' : 00 00