## 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;