use v6;
my $data = q[colname1 colname2 colname3
  val1     val2        val3
  val11    val21      val31
  val12     val22       val32];

my @lines = $data.split: "\n";
my @rows;
my @header = @lines.shift.words;
for @lines -> $l {
    my %h = @header Z $l.words;
    @rows.push: {%h};
}
say @rows.perl;
my @colname = <colname1 colname3>;

say @colname.join("\t");
say $_{@colname}.join("\t") for @rows;
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Though I admit that the output could be formatted a bit prettier, and some parts of the code are ugly hacks.