dave_the_m's example leads to a very practical, if slightly contrived use for double declaration.

You have a sub that returns two subrefs. Those two subrefs are, as you might assume from their being instantiated at the same time, related. And they both rely upon a variable of some domain specific name that they inherit through closure. But they each need a different one:

sub generator {
    ...
    my $frobnosticator = 0;
    my $coderef1 = sub{ ...; $frobnosticator ^= 1; ... };

    my $frobnosticator = 0;
    my $coderef1 = sub{ ...; $frobnosticator ^= 1; ... };

    return $coderef1, $coderef2;
}
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There, perfectly sensible :)