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";