perl -MData::Dump -e '$val="(".("{(a,1),(b,2)},"x2).")";@hash{(key1,key2)} = eval $val;dd $val;dd \%hash'
"({(a,1),(b,2)},{(a,1),(b,2)},)"
{ key1 => { a => 1, b => 2 }, key2 => { a => 1, b => 2 } }

# Or more concisely:
perl -MData::Dump -e '@hash{(key1,key2)} = eval "(".("{(a,1),(b,2)},"x2).")";dd \%hash'