1) I have no preference (even if slice might Benchmark faster), its all the same , but I like to online the slice

my %foo; @foo{@bar} = (0) x @bar;

Just because its not legal to  my( @foo{ @bar } ) = (0) x @bar; doesn't mean it can't be onelined :)

2) This one is trickier, both of those irritate me slightly, though not enough prefer either, or not-use either :)

Out of habit, unless my editor helps out with whitespace, I multi-line my ternary

my $foo = 42 == $bar
          ? ".."
          : "";
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Although I actually prefer

my $foo = ""; 42==$bar and $foo = "forty two";

I'm fine with  my $foo = ""; $foo = "forty two" if 42==$bar; too

All the choices are very much a non issue for me, even if the slightly irritating versions break/disrupt flow/scanability/skimmability for some, like inline comments ( Documentation: POD vs Comments), I doesn't slow me down much , its mosquito or hurdle not doors/walls