It's the scoping of the parentheses around the my assignment that causes the problem.

The parens have nothing to do with it. A 'my' declaration has no effect until the statement after the 'my'. This is, at least in part, so you can do:

my $x = 2;
{
    my $x = 3 * $x;
    # $x == 6
}
# $x == 2
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As to your question, you can get away with:

(@$_ = keys %foo) == 1 or die "Got more than one: @$_\n" for \my @v;
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But I'm not sure it is worth it.