That my Linux box (v5.8.8 built for x86_64-linux-gnu-thread-multi under Ubuntu) replicates the issue and my Windows box (v5.8.9 built for MSWin32-x86-multi-thread, I forgot I upgraded) does not implies something differs between the implementations. Running the Linux variety through my debugger shows $line to be undef on line 7 on the continuation case, whereas it maintains continuity under AS. Since we are dealing with a variable which is scoped at the loop, I was reading it as the variable was getting a new instance upon redo even though the my was not being re-evaluated. This is supported by the following code, which prints an infinite series of 1's on my Windows box and prints one 1 and then an infinite series of "Use of uninitialized value in print at fluff.pl line 6, <DATA> line 1." on my Linux box:

#!/usr/bin/perl -w
use strict;
use warnings;

while (defined(my $line = <DATA>) ) {
    print $line;
    redo;
}

__DATA__
1
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Update: Better example code (I think). Prints 210 under Windows and 2 under Linux:

#!/usr/bin/perl
use strict;
use warnings;

my $j = 2;
while ( my $i = $j ) {
    print $i if defined $i;
    last unless $i--;
    redo;
}
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Update 2: Filed a bug report, id [perl #63752]