use Data::Dumper;

my @ary_of_hrefs = (
    { id_1 => 'a', id_2 => 'b', f1 => 'p', f2 => 'i' },
    { id_1 => 'a', id_2 => 'c', f1 => 'e', f2 => 'm' },
    { id_1 => 'b', id_2 => 'b', f1 => 'r', f2 => 'h' },
    { id_1 => 'b', id_2 => 'c', f1 => 'l', f2 => 'o' }
);

my %hash;

map {
    $hash{ delete $_->{id_1} }{ delete $_->{id_2} } = $_
} @ary_of_hrefs;

print Dumper(\%hash), "\n";


##</code><code>##

$VAR1 = {
          'a' => {
                   'b' => {
                            'f1' => 'p',
                            'f2' => 'i'
                          },
                   'c' => {
                            'f1' => 'e',
                            'f2' => 'm'
                          }
                 },
          'b' => {
                   'b' => {
                            'f1' => 'r',
                            'f2' => 'h'
                          },
                   'c' => {
                            'f1' => 'l',
                            'f2' => 'o'
                          }
                 }
        };


##</code><code>##

for ( @ary_of_hrefs ) {
    $hash{ delete $_->{id_1} }{ delete $_->{id_2} } = $_;
}