$a = [
  {
    a => a1,
    b => b1,
    c => c1,
  },
  {
    a => a2,
    b => b2,
    c => c2,
  },
  {
    a => a3,
    b => b3,
  }
]

$b = $a.map |$h| {
  $c = $h[c] ? {
    undef   => '',
    default => ":${h[c]}"
  }
  "-v ${h[a]}:${h[b]}${c}"
}.join(' ')

notice($b)

# -v a1:b1:c1 -v a2:b2:c2