xxd-o

#!/usr/bin/env perl
use IPC::Open2; 
$SIG{'__WARN__'} = sub{ die "$0: $!\n" };

my $offset = shift // "0";
$offset = oct $offset if $offset =~ /^0/;
my $base = $offset >= 2**32 ? 16 : 8;

my $zeroes = $offset % 16;
my $padding = 1 + int($zeroes / 2) + 2*$zeroes;
my $bytestr = "\0" x $zeroes;
{ local $/; $bytestr .= <> }

open2(\*XXD_OUT, \*XXD_IN, "xxd") or die "xxd is not available!";
print XXD_IN $bytestr; close XXD_IN;

if ($zeroes) {
	$_ = <XXD_OUT>;
	s/^(.{50}).{$zeroes}/$1 . (' ' x $zeroes)/ge;
	s/^([[:xdigit:]]+:).{$padding}/$1 . (' ' x $padding)/ge;
	my $newoff = sprintf("%0${base}x",hex($1)+$offset) =~ s/^(.{8})(.{8})$/$1_$2/r;
	s/^([[:xdigit:]]+):/$newoff:/g;
	print
}
while (<XXD_OUT>) {
	s/^([[:xdigit:]]+)(?=:)/sprintf("%0${base}x", hex($1)+$offset-$offset%16) =~ s[^(.{8})(.{8})$][$1_$2]r/ge;
	print
}