#!/usr/bin/perl -wl
use strict;

my $little;
BEGIN { $little= unpack "C", pack "S", 1; }

sub quad {
    my( $str )= @_;
    my $big;
    if(  ! eval { $big= unpack( "Q", $str ); 1; }  ) {
        my( $lo, $hi )= unpack "LL", $str;
        ( $hi, $lo )= ( $lo, $hi )   if  ! $little;
        $big= $lo + $hi*( 1 + ~0 );
        if(  $big+1 == $big  ) {
            warn "Forced to approximate!\n";
        }
    }
    return $big;
}

my @quads= (
    "\xff\xff\xff\xff\xff\xff\xff\xff",
    "\x00\x3f\xff\xff\xff\xff\xff\xff",
    "\x00\x1f\xff\xff\xff\xff\xff\xff" );

if(  $little  ) {
    for(  @quads  ) {
        $_= reverse $_;
    }
}
for(  @quads  ) {
    print quad( $_ );
}
__END__
This prints:
    Forced to approximate!
    1.84467440737096e+019
    Forced to approximate!
    1.8014398509482e+016
    9.00719925474099e+015