Reducing the test case for Perl 5.6.1:

$_ = <<EOF;
1a
2a
EOF

@a = /(.*?)a/sg;
print "@a\n";

# prints:
# 1

@a = /([\000-\377]*?)a/g;
print "@a\n";

# prints:
# 1
# 2
[download]

Update:

Looks like this is a known bug, ID 20010410.006 on http://bugs.perl.org/