Same code with expanded regex with comments

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1224600

use strict;
use warnings;
use Data::Dump 'dd';

my $data = <<'END';
interface XYZ
  given param1 -> child of "interface XYZ"
  given param2 -> child of "interface XYZ"
    given param2.1 -> child of "given param2"
      given param2.1.1 -> child of "given param2.1"
      given param2.1.2 -> child of "given param2.1"
    given param2.2 -> child of "given param2"
  given param3 -> child of "interface XYZ"
  given param4 -> child of "interface XYZ"
interface SECOND
  given param5 -> child of "interface SECOND"
END

my $struct = buildstruct($data);
dd $struct;

sub buildstruct
  {
  my $block = shift;
  my @answers;
  while( $block =~ /^ # make sure to start at beginning of a line (wit
+h m)
      (\ *)           # match leading spaces of header line
      (.*)            # match rest of line, save as head
      \n              # and match the newline
      (
      (?:             # match all following lines with
        \1            # same whitespace as head
        \ +           # plus at least one more space ( i.e. indented )
        .*\n          # contents to be looked at later
      )*              # as many as possible
      )               # save as rest
      /gmx )          # global, multiline, and extra whitespace
    {
    my ($head, $rest) = ($2, $3);
    $head =~ s/ ->.*//;
    push @answers, $rest ? { $head => buildstruct($rest) } : $head;
    }
  \@answers;
  }
[download]

I hope this helps :)

The regex matches each line, gets the indentation space string, then also matches all following lines that are indented that much plus at least one more space.