use strict;
use warnings;
use Data::Dumper 'Dumper';

my %tree;
while (<DATA>) {
    chomp;
    next unless ( /^\s*(\d+)\s*=>\s*(.*)/ );
    my ( $key, $val ) = ( $1, $2 );
    if ( $val eq '' ) {  # leaf node
        $tree{$key} = {} unless exists( $tree{$key} );  # assign an empty hash ref
    }
    else {
        for my $node ( split /, +/, $val ) {
	    if ( ! exists( $tree{$node} )) {
		$tree{$node} = {};
	    }
	    $tree{$key}{$node} = \$tree{$node};
	    push @{$tree{$node}{parent}}, $key; # keep track of node parents
        }
    }
}

# all done:
for ( sort keys %tree ) {
    print "======\n$_:\n", Dumper( $tree{$_} ) unless exists( $tree{$_}{parent} );
}

# append your data below...
__DATA__