use List::Util qw[ reduce ];

my $line = 'a1:a2:a3=key';
my $href = reduce{ my $r={}; $r->{$b}=$a; $r } reverse split /:|=/, $line;
print Dumper $href;
$VAR1 = {
          'a1' => {
                    'a2' => {
                              'a3' => 'key'
                            }
                  }
        };