Okay, I think I have signed quads working...

#!/usr/bin/perl -w
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;
}

sub squad {
    my( $str )= @_;
    my $big;
    if(  ! eval { $big= unpack( "Q", $str ); 1; }  ) {
        my( $lo, $hi )= unpack $little ? "Ll" : "lL", $str;
        ( $hi, $lo )= ( $lo, $hi )   if  ! $little;
        if(  $hi < 0  ) {
            $hi= ~$hi;
            $lo= ~$lo;
            $big= -1 -$lo - $hi*( 1 + ~0 );
        } else {
            $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\xed\xcb\xa9\x87\x65\x43",
    "\x00\x1f\xff\xff\xff\xff\xff\xff" );

if(  $little  ) {
    for(  @quads  ) {
        $_= reverse $_;
    }
}
$|= 1;
push @quads, ~$quads[1], ~$quads[2];
for(  @quads  ) {
    print quad( $_ ), "\n";
    print squad( $_ ), "\n\n";
}
if(  quad($quads[2]) == -1 - squad($quads[4])  ) {
    print "two's complement works!\n";
}
__END__
This prints:
    Forced to approximate!
    1.84467440737096e+019
    -1

    Forced to approximate!
    1.79943825111381e+016
    Forced to approximate!
    1.79943825111381e+016

    9.00719925474099e+015
    9.00719925474099e+015

    Forced to approximate!
    1.84287496911984e+019
    Forced to approximate!
    -1.79943825111381e+016

    Forced to approximate!
    1.84377368744548e+019
    -9.00719925474099e+015

    two's complement works!
[download]

Though, squad() seems overly convoluted. Improvements welcome.