++sn1987a (when the Vote Fairy next visits) for an ingenious observation! Unfortunately, I don’t think it answers the OP’s question, as masking with bitwise-AND does remove the sign bit, even with use integer in effect:

#! perl
use strict;
use warnings;

print "\nno integer\n";

my $bit = -1 >> 31;
printf "unmasked: 0x%x = %d\n", $bit, $bit;

$bit = (-1 & 0x80000000) >> 31;
printf "masked:   0x%x = %d\n", $bit, $bit;

use integer;

print "\nuse integer\n";

$bit = -1 >> 31;
printf "unmasked: 0x%x = %d\n", $bit, $bit;

$bit = (-1 & 0x80000000) >> 31;
printf "masked:   0x%x = %d\n", $bit, $bit;
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Output:

 1:29 >perl 868_SoPW.pl

no integer
unmasked: 0x1ffffffff = 8589934591
masked:   0x1 = 1

use integer
unmasked: 0xffffffffffffffff = -1
masked:   0x1 = 1

 1:29 >perl -v

This is perl 5, version 18, subversion 2 (v5.18.2) built for MSWin32-x
+86-multi-thread-64int
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:-(

Update: As BrowserUk observes below, masking removes the sign bit (in this case) only in perls built with 64-bit ints. For perls built with 32-bit ints, sn1987a’s observation does answer the OP’s question. (Moral for self: test, test, test!)

built for MSWin32-x86-multi-thread-64int

The problem only(*) occurs with 32-bit perls:

C:\test>type CIDRtest.pl
#! perl -slw
use strict;
use integer;

for my $n ( 0 .. 31 ) {
    my $addr = (1 << $n) + 1;
    printf "\r%u", $addr;
    my $nbits = 32;
    while( 1 ) {
        my $bit = ( $addr & 0x80000000 ) >> 31;
        die "$addr -> $bit" if $bit != 0 and $bit != 1;
        $addr <<= 1;
        last unless --$nbits;
    }
}

C:\test>CIDRtest.pl
2147483649

C:\test>\perl32\bin\perl.exe CIDRtest.pl
-2147483648 -> -1 at CIDRtest.pl line 11.
2
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(*Or 64-bit perls, but with much higher numbers.)

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

built for MSWin32-x86-multi-thread-64int

The problem only(*) occurs with 32-bit perls:

Aaargh ... and hadn't even noticed that my laptop's system perl has 64 bit ints even though it's i686 so I'd used it for testingwhat I thought was a 32 bit system m(

Many thanks everybody, that was the whack I needed!

Thanks for the update Athanasius. I was not aware of that feature. :)

Oh well, back to the drawing board...