Update: Simplified the packing by removing a redundant step.

Update2: eliminated the map from the packing.

Non-trivial:

#! perl -slw
use strict;
use Data::Dump qw[ pp ];
use Math::Random::MT qw[ rand ];

my @sint24s = map{ -2**23 + int( rand 2**24 ) } 1 .. 20;

## removed duplicated my $packed -- Thanks to AnonmalousMonk
my $packed = join '', unpack '(a3x)*', pack 'l*', @sint24s;

my @unpacked = map {
    unpack 'l', $_ . chr( vec( $_, 23, 1 ) ? 255 : 0 );
} unpack '(a3)*', $packed;

print "$sint24s[ $_ ] ;; $unpacked[ $_ ]" for 0 .. $#sint24s;


__END__
C:\test>sint24
4243386 ;; 4243386
4809369 ;; 4809369
-888567 ;; -888567
-7576685 ;; -7576685
1987080 ;; 1987080
-2170022 ;; -2170022
-1135866 ;; -1135866
1924446 ;; 1924446
6348263 ;; 6348263
1911716 ;; 1911716
-1791354 ;; -1791354
-8343943 ;; -8343943
-6224088 ;; -6224088
3919567 ;; 3919567
-1176382 ;; -1176382
6288012 ;; 6288012
-5569609 ;; -5569609
-5363232 ;; -5363232
-1344267 ;; -1344267
3649155 ;; 3649155
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