Esp. syntax #2 looks a little overcomplicated, why not:

sub f {
    length or $_ = "default value" for
        my $var = shift;
    do_something_with($var);
}
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But in my humble opinion I'd prefer for the sake of clarity:

sub f {
    my $var = shift;
    for ( $var ) {
        last if length;
        $_ = $DEFAULT;
    }
    ...
}
[download]