>>811
TMTOWTDI.

# substr
($A, $B) = (substr($A, 2), substr($A, 0, 2));

# unpack
($B, $A) = unpack 'A2A*', $A;

# 置換1
$A =~ s/^(..)//;
$B = $1;

# 置換2
$A =~ s/^(..)/($B = $1, '')[1]/e;

# 回りくどい方法
@tmp = split //, $A;
$B = join '', splice @tmp, 0, 2;
$A = join '', @tmp;