my( $width, $height ) = unpack 'I2', substr $data, $offset, 8;

##</code><code>##

print "$width : $height";
327680 : 262144;

##</code><code>##

printf "%u : %u\n", unpack 'V2', substr $data, $offset, 8;
1280 : 1024

##</code><code>##

## Assume the first 8 bytes are a double in the 'other' endianess. 
##  And the next 4 a float.
my $data = ...; 

my $doublePackedOtherEndian = substr $data, 0, 8;
my $floatPackedOtherEndian  = substr $data, 9, 4;

## Now reverse the bytes
my $doublePackMyEndian  = reverse $doublePackedOtherEndian;
my $floatPackedMyEndian = reverse my $floatPackedOtherEndian;

## And unpack
my $double = unpack 'd', $doublePackMyEndian;
my $float  = unpack 'f', $floatPackedMyEndian;