use List::Util 'reduce';
use Data::Dumper;

$_ = "a1:a2:a3=foo";

my ($key,$val) = split(/=/);

my @keys = split /:/, $key;
my $last = pop @keys;

my %hash;
( @keys
  ? reduce( sub { $a->{$b} ||= {} },
            \%hash,
            @keys )
  : \%hash )->{ $last } = $val;

print Dumper( \%hash );