You could eliminate the loop pretty easily.

#!/usr/bin/perl

use strict;
use warnings 'all';
use Config;

my $longsize = $Config{longsize};
my $bits = $longsize * 8;

sub rshift { # Should be signed/sign-propagating right shift
    my ($x, $y) = @_;
    my $mask = 0; 
    my $z = $x >> $y;
    $mask = (2 ** ($y + 1) -1) << ($bits - $y) if $x < 0;
    return $z | $mask;
}
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It won't be very easy to turn it into an infix operator in Perl 5 without using a source filter... and you really don't want to go there if you don't have to.

UPDATE:Or if you want to calculate native size a different way:

#!/usr/bin/perl

use strict;
use warnings 'all';

my $bits;
{
    my $native = -1;
    $bits++ while $native >>= 1;
} 

sub rshift { # Should be signed/sign-propagating right shift
    my ($x, $y) = @_;
    my $mask = 0; 
    my $z = $x >> $y;
    $mask = ( -1 >> ($bits - $y) ) << ($bits - $y) if $x < 0;
    return $z | $mask;
}
[download]