I think the following code is self explaining

use strict;
use warnings;
use Data::Dump qw/pp dd/;


my @parse;
my @path;

my $last_level=0;
$path[$last_level] = \@parse;



while (my $line = <DATA> ) {
    # pp
    my($white,$key,undef,$content) =
      $line =~ /^
                (\s*)                   # indent
                (.*?)                   # key
                (
                    \s*->\s*            # ignore arrow
                    (.*)
                )?                      # optional group
                $/x;

    my $level = length($white) / 2;

    # pp [$white, $key,$level,$last_level];

    die "indent-level $level too big (last level was $last_level)!"
      if $level > $last_level+1;


    my @children;

    push
      @{$path[$level]},
      # {
      #  $key => {
      #           children => \@children,
      #           # level => $level,
      #           # content => $content,
      #          }
      # };
        {
         $key => \@children      # terse output
        };

    $path[$level+1] = \@children;
    $last_level = $level;

}

warn "Output: " , pp \@parse;


__DATA__
interface XYZ
  given param1 -> child of "interface XYZ"
  given param2 -> child of "interface XYZ"
    given param2.1 -> child of "given param2"
    given param2.2 -> child of "given param2"
      given param2.2.1 -> child of "given param2.2"
  given param3 -> child of "interface XYZ"
[download]

I extended your input to cover the case of a bigger indent gap.

Output: [
  {
    "interface XYZ" => [
      { "given param1" => [] },
      {
        "given param2" => [
          { "given param2.1" => [] },
          { "given param2.2" => [{ "given param2.2.1" => [] }] },
        ],
      },
      { "given param3" => [] },
    ],
  },
] at d:/tmp/parse_indent.pl line 55, <DATA> line 7.
[download]

you can uncomment various code sections to play with debug output and different data-structure patterns YMMV.

update

Some people prefer to avoid empty "children" arrays for leafs-nodes.

In this case avoid an empty default array and let $path[$level] point to an upper container where you check for existence of an entry for children.

extending the code should be straight forward.

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