#! /usr/bin/env perl
use strict;
use warnings;


# 32bit data
my $buf = 0x12345678;

# reverse byte order in 32bit value
my $buf2  = unpack( 'L', reverse pack L => $buf );

# show me the bytes in hex:
my $FORMAT = ( "%02x " x 4 ) . "\n";
printf $FORMAT, unpack( "W*", pack L => $buf  );
printf $FORMAT, unpack( "W*", pack L => $buf2 );

__END__
Output:
78 56 34 12
12 34 56 78